The keyboard somehow loses control of the active window - whether mixer or editor window, so basically key commands are carried out in the background (ie, the arrange window) even when the mixer or edit window is active. Mouse works fine, but key commands don't.
This means that a simple mouse selection of notes in the EDIT window, or God forbid, a 'select all' command followed by 'delete' key selects all and deletes all items in the ARRANGE window (which could be invisible behind the edit window!!!). DANGEROUS! Also, if you are like me - very quick with key commands as part of your workflow, you could be carrying out a sequence of key commands in the background before you realise what's happening and it becomes difficult to re-trace your steps, not to mention EXTREMELY ANNOYING!
Also, as some others have pointed out, you lose the ability to name channels in the mixer when this happens - the mixer is expecting some typing, but you get a 'error' beep because it 'thinks' you are trying to access other windows instead of replacing the highlighted text.
Closing windows and re-opening will remedy, but not for long - it comes back over and over during a session.
PLEASE, Presonus sort this out. I don't get how an update makes the software worse??
Mac Pro Mid 2010 12 core 2.66GHz, Mojave 10.14.6, S1 Pro 5.2, dual display.