Upon further investigation, we've found the following:
Premiere Pro 15.0 usually (80% of the time) doesn't recognize plugins contained in /Library/Application Support/Adobe/Common/Plug-ins/ControlSurface/ -- this plugin directory worked with Premiere versions 12.x -> 14.x and is the default install directory for the Monogram plugin.
Premiere crashes if a plugin lives in the above directory and the user tries to access Preferences > Control Surface
A temporary workaround is to move the plugin file to /Applications/Adobe Premiere Pro 2021/Adobe Premiere Pro 2021.app/Contents/Plug-Ins/ControlSurface
The workaround outlined above works on a case-by-case basis, but Premiere Pro really should be patched to accommodate plugins inside the Common directory as previous versions have.