LMS 7.9.1 scenario to sync 3 Player - issues......

Hi all,
it won`t be long and I´m going cracy - would be great if someone can help me out.

I do have LMS 7.9.1 running on a Synology NAS.

I believe I had the following scenario anytime in the past already running - but not sure.
I can´t figure out how to run following scenario with 2 Touch and one SB player:
Touch A and Touch B should be synchronized and power on/off together.
SB C should be synchronized, but powered on/off individual when needed.

Settings in LMS:
Touch A: Synchronize with Touch B & C -- Power on/off: In Group
Touch B: Synchronize with Touch A & C -- Power on/off: In Gruop
SB C: Synchronize with Touch A & B -- Power on/off: independent

If I am only using Touch A&B all works like expected.
When in addition I power on "SB C" all works like expected -- BUT for whatever reason when I power off "SB C" the Touch A&B also power off - why??
What settings are wrong - or do I have to apply the settings in a special order?

Thanks so much in advance for your support