Posting this in the main thread for more visibility, as it gets lost it other lower subcomments:
So the issue with inactive windows of Office apps like Outlook going blank white in RDP/Citrix sessions appears to be caused by InkObj.dll, located at "C:\Program Files (x86)\Common Files\microsoft shared\Ink" for x86 and "C:\Program Files\Common Files\microsoft shared\Ink" on x64. Taking ownership of and renaming or otherwise removing this .dll fixes the issue, however on 64 bit systems it breaks the Snipping Tool which also relies on this .dll file
Noticing that when InkObj.dll was not loaded that Outlook no longer showed the "Draw" feature, I stumbled upon a fix for the Office white screen problem that doesn't break the Snipping Tool: In Outlook (where we have had the problem) open up an email or compose a new one, then go File -> Options -> Customize Ribbon and disable the "Draw" ribbon feature under main tabs
With the Draw ribbon item disabled for open or composing windows, inactive windows no longer turn white. Can also confirm it's the same fix in other Office apps like Word