Yep, same problem here - and lots of others mentioning this.! This is a serious bug which is dangerous if you have a fast workflow...
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!
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, Studio One Pro 5.2, two monitors.