I noticed that JCE 2.9.9 pro removes this attribute from image tag: group="gallery"
When I open some older articles it strips it as well, so it's not on save.
If I turn off editor after page article save or page refresh it auto tuns editor on.
Steps to reproduce: Turn editor off by pressing power button, refresh page, editor will be turned on again.
After reverting to 2.9.8 it works OK again, without stripping attribute
Joomla 3.9.27, Filtering off, Allowing all code block types in JCE profiles