I've been having this issue lately, amongst others. I've made a video that hopefully explains the problem.

I thought it might be to do with negative scaling, but when I made the video it still affected non scaled bones, though it seemed to be worse or different with the negatively scaled bone.

In the video I have three bones with the same animation, the top one is the base, the middle one has been scaled by -1 in the X axis under the Setup mode, and the bottom one is the same as the top, except it's been scaled by -1 on the X axis with a keyframe on the first frame.

The bug is most obvious 25 seconds in on the bottom bone.

Additionally at the end I show there's still a bug present where if you start typing in the Y of the shear input box, the cursor immediately jumps to the left, making it very difficult to input shear values.

I'm using 3.6.52 essential.

There is also a minor issue in that Spine is the only program that does not respond to my button macros.

I have a logitch G105 which has programmable buttons on the left side. Normally I have these set to do ctrl + C, X and V for copy/cut/paste. Spine is the only program that doesn't respond to this at all, for some reason.

It may be an issue with keyboard regions? Normally I have my keyboard set to the Dvorak layout, in an attempt to reduce likelihood of developing RSI, but I switch to QWERTY when using programs like Photoshop or Spine.