Version 3.4.01
Importing data in either json or binary formats to spine results in the default spacing mode being used for all path constraints.
Create a spine project with a path + bone(s).
Add a path constraint, set spacing mode to percent.
Export the project in either json or binary format. (settings used: no atlas, nonessential, pretty print for json)
Import the skeleton into a new project. (Default scale, new project, tested with both binary and json)
The spacing mode will be set to length.
The exported json file does state the correct spacing mode, so it looks like this issue related to importing.