You want to be using Bounding Box attached to a bone instead:
https://github.com/gritsenko/c3_spine_plugin#bounding-box-attachment
Then you can use the bounding box center expressions to track where the bone is.
See the bounding box example for details (don't forget to turn off worker mode for that project though!)
Additional updates
Oh btw the test object didn't work with the new version of the plugin until I added a Bounding Box. Maybe it's a coincidence.
**edit: Nvm it was the Enable Worker thing. Main project runs beautifully now <3
Anyway, I did the Bounding Box and it didn't help, particles still reset to 0,0. I'm probably doing something incredibly stupid, but I have no idea what D:
1.50.2 Fix for worker mode now released.
https://github.com/gritsenko/c3_spine_plugin/releases/tag/1.50.2
For the bounding box are you doing the UpdateBBoxes ACE before calling the expressions? Also, see the example project.
From the README.md:
SpineBBoxCenterX, SpineBBoxCenterY expressions give the average of the named slot/bbox polygon points. Useful for attaching C3 Sprite object/collision box to the center of the Bounding Box.
The UpdateBBoxes updates the bounding box to the current point in the animation. It should be done just once per tick per instance before the SpineBBoxCenterX/Y expressions are used.
Also check the dev console, do you see any errors or warnings?
NOW IT WORKS!!! kinda... Thank you <3
Here, you can see the experiment.
https://porcupine-parkour.hr/goblinwave/
As you might have noticed, I'm more of an animator, not really a programmer, so I get hung up on basic things 
There is also a fair number of errors and they seem to be connected to BBox
Also, I found this in the example project, it's going to be very helpful in the future since I tend to resize things a lot.
Getting there! If you want to send me the project I can take a look. You can DM me on twitter or on the Construct Community Discord (Mikal). I use sendgb.com to share project files usually.
Spine 4.0 beta test ongoing, you must export from Spine 4.0 (3.8 exports will not work.)
If you already have experience with Spine and the C3 plug-in, try it out. If you have not used the C3 plug-in before, I recommend staying with the current 3.8 version as the new 4.0 beta may have bugs.
4.0 beta:
https://github.com/gritsenko/c3_spine_plugin#spine-40x-compatible-addon-beta-version
Spine 4.1 support added. Yay! Sequence animation support!
Updated to add spine-ts 4.1.19 runtime, also added Load Files ACE
See updates here:
https://github.com/gritsenko/c3_spine_plugin
Release 2.5.0
Add zElevation standard property and ACEs.
first of all, great job!
Secondly, In JS runtime you can change any slot skins in runtime by creating new custom skin or even replacing the single slot.
( https://github.com/pixijs/spine/blob/master/examples/change_skin.md )
There is a way to do that in construct? (except from use the js sdk directly)
Case of use: a character with different skin and I would like to mix-and-match slots
You can, check out the ACEs for create new skin, add skin and apply skin.
Update:
https://github.com/gritsenko/c3_spine_plugin/releases/tag/2.8.2
Fix C3 324 compat and set region ACE for 4.1
Interesting information
Update to runtime 4.2 (physics, pixel mode)
Please review: gritsenko/c3_spine_pluginreleases/tag/2.9.0
Thanks to the donation, I sped up work on this.
Please test, I have not done much with physics myself, so I don't what additional ACEs (if any) are needed.
I did add the ACE Set physics mode w/ the default being 'update'
If you have a physics project to share, post a link here or send me a DM on Construct Community Discord server, so I can help further test.
Noticed one change needed in our work
@MikalDev i was able to access the skeleton json and just manually parse it, but thank you
anyway, if you or anyone else could help:
i'm playing around with the mix and match sample spine project. can we dynamically create a spine object? i tried but it seems the skin isn't changing
how do i resize a spine object (skin) so that the whole thing is within the bounding box? set size allows me to change the size, but it doesn't guarantee the whole skin is displayed and not cropped out
2.9.1: Fix for physics not updating
thank you, here's the link to the sample project file i'm trying to play around with: https://sendgb.com/sJzyiRp7vYN
basically, i'd like to be able to dynamically create a new spine object during runtime each with a different "skin" (example: backpack, bag, scarf, etc.) when i press A
i dragged 1 spine object in the editor and set the default skin to "accessories/backpack"
during runtime, when i press A, i create an object then create and set a custom skin "accessories/scarf", but somehow the new spine object created still has the original skin i set in the layout editor which is the backpack. i've also tried directly doing "set skin accessories/scarf" instead of creating a custom skin, got the same result.
@MikalDev
oh wow thank you for this, finally got it working
i still haven't completely figured out fixing the cropped image if the initial skin for a spine object is smaller than the skin set for a new dynamically created spine object
kindly check if you don't mind: sendgb.com/JQ9lX5TnLAr
thank you
sorry for double posting in two forums, take a while for the mods to approve my post in the construct forum
@MikalDev any hopes to get an update that fixes support for the playable ad C3 export? When you export as a single file with base64 assets, the plugin can load the PNG for Spine from the base64, but the .atlas and .json always 404 as the atlasURI and jsonURI are trying to look for the actual file itself instead of using the base64 string in self.c3_base64files.
