This issue is caused by an older BEAM plugin file still being present. When this happens, your DAW can keep pointing old project instances at the old path, so they show “VST3 plug-in could not be found.”
To fix this issue:
Delete the following folders while being sure to backup favorites.json and the User folder to save any favorited patches and user presets respectively:
Windows
VST3: C:\Program Files\Common Files\VST3\BEAM.vst3
AAX: C:\Program Files\Common Files\Avid\Audio\Plug-Ins\BEAM.aaxplugin
Presets and Settings: {USER}\AppData\Roaming\Lunacy Audio\BEAMMac
VST3: /Library/Audio/Plug-Ins/VST3/BEAM.vst3
AU: /Library/Audio/Plug-Ins/Components/BEAM.component
AAX: /Library/Application Support/Avid/Audio/BEAM.aaxplugin
Presets and Settings: Users/{USERNAME}/Library/Application Support/Lunacy Audio/BEAM
Reinstall BEAM by visiting your Downloads page located here to download and install the latest version of BEAM.