Unfortuntaly it turned out that it was not only a problem in the Spine-Unity runtime. Responsibilities have been shifted between programmers, which always costs some additional overhead, in addition to a very busy week. We are sorry for the inconvenience.
It turned out that this is not a bug but behaviour as intended (related to your constraint setup), please see the comment by Nate in the issue ticket here:
[runtimes] Scale of -1 results in incorrect bone orientation · #1373
