It is OBS (I see the typo that I did in the OP) (Open Broadcaster Software) used to mix in multiple video and audio inputs to be used to stream/record content.
I can try to duplicate it using a game executed from Launch Box and MAME64 instead of going through MAMEUI64 to see if the same thing happens or not.
And if I can't get that figured out, I'll work out a test running MAME via command line (I've not tried doing that in over 20 years )