Directory Opus Pro 12.22 Build 7593 x64
OS 10.0 (B:19041 P:2 T:1) SP 0.0
See the attached video.
I go to Customize mode, uncheck Start group for "Odstęp" spacer and accept change.
Then I go back to Customize mode and the option is back.
This seems to happen not only for spacer, and it seems to me sometimes when I set Start group for a toolbar element, it is set for a neighbouring item instead.