Not sure if I've found a solution, or just a good temporary fix.
The compatibility/run as flags are saved somewhere in the registry. But I'm no expert with the registry, so I found a utility called AppCompatibilityView from Nirsoft. https://www.nirsoft.net/utils/app_compatibility_view.html
It showed all of the programs/games in my system that have compatibility settings set, and Rundll32.exe was in the list. From there, I was able to select the Rundll32.exe entry, then go 'Action' > 'Delete Selected Item' to remove the compatibility settings for the item.
Been 3 months without the issue coming back, so far anyway. 🤞
Still not sure what caused it to get set that way in the first place.