Adobe Reader DC IE Plugin - Process Stays Running on Windows Server 2019

We are in the process of deploying a new RDS (Remote Desktop Services) farm, running on Windows Server 2019 Datacenter. Tests have mostly gone well, but there has been an issue highligted with some of the browser based systems that we use that display PDF's within the browser (I think as iframe's, but I don't know for sure, as they are not systems we have access to source code for). This launches AcroRd32.exe as a process within IE, but sometimes, when launching the next PDF, we are seeing a variety of errors, ranging from grey screens in IE to Error 523:523 in some cases. From observation it seems that the process (which sometimes also includes RdrCEF.exe) are still running in the background.

I have tested with both v1902120047 (which we run in prodction) and also the latest, v2000620042 which seem to both have similar issues. I cannot replicate the same issue on Windows 10 from what I have tried.

The rogue processes usually disappear in one of a few ways:

• They time out and just go as quick as they came
• They generate an error about being unable to contact a DDE Server (which once acknowledged, clears the process)
• Opening Adobe Reader itself often seems to clear them up
• Worst case scenario is killing the tasks from Task Manager

I have been spinning up various Server 2019 VM's to try and get to the bottom of the issue, without success, each with variations in the configuration (mainly what software is installed). In the most recent case I have spun up a Server 2019 VM (domain joined) which has no AV, no firewall and no other software or roles installed, other than Adobe Reader DC, so I don't believe it is any of our custom configs causing the issue.

The only thing that does reliably "fix" it, is disabling Protected Mode (which then opens the PDF's in AcroBroker.exe instead) - but of course this "fix" is not without its' problems

Has anyone come across this, found a fix or could this just be a bug in Adobe Reader DC?