So the limitation appeared after upgrade to 81.10.17. ? Did you try to revert to the former version ?
What if you uncheck the Limit the number.. setting ? Then it should be unlimited...
With same firmware version on both nodes all should be OK. I just wonder why you differentiate between active and standby member - usually, you only connect to the VIP (that is the active node) and only do configuration changes there, as the standby will sync it automatically.
I would suggest to contact CP TAC to get behind this!
CCSP - CCSE / CCTE / CTPS / CCME / CCSM Elite / SMB Specialist