Mask sensitive visibility query?

Hi all, don't know where to post this (Haven't had an issue with classic for a while)

I have a combat running with client connected.
I have the client view locked and monsters invisible
I start to set the monsters as mask sensitive, but the first monster I set as Mask sensitive (which is outside of the unmasked area) shows up on the client's screen.
See screenshot Attachment 35671
The other monster the client can see is mask sensitive also (the rest are still invisible).
Probably missing something obvious as I have been on FGU recently :p

Am running 4E D&D with the latest Enhanced images extension. The Characters are on the correct layer
Rebooted and tried several campaigns
Other extensions too, but they have not been an issue in the past.
Just tried with a vanilla campaign (no extensions enabled) Attachment 35672
and 5E D&D.
The only way a Monster outside the mask is Mask sensitive (invisible to the client) is if Enhanced Images is disabled. Attachment 35673