I found that removing the following folder caused Logic Pro X to perform a rescan of plug-ins when it starts:
/Users/your_username_here/Library/Caches/AudioUnitCache/
I’m on macOS High Sierra 10.13.6 with Logic Pro X 10.4.1
I don’t know if this helps in making Logic see newly installed plug-in, but this was certainly beneficial in making Logic understand a change of name in the plug-in:
A previous build of my plug-in was named “OldName” and was seen as such by Logic.
Then I renamed the plug-in project to “NewName” leaving the IDs (aufx ABCD MyPl) and filename intact and still Logic listed it as “OldName”, a sign that it had cached “OldName” in association with those IDs
I switched to “Guest User” on my machine and started Logic there. Under that user, it showed the correct name “NewName” for the plug-in, so that got me convinced that the cache was stored on a per-user basis.
I switched back to my regular user account and started looking around in ~/Library/Caches/ which sounded promising, and pronto I found that AudioUnitCache folder.
I deleted it and, when I restarted Logic, I got the small notification that it did a rescan of the AUs. After that, the plug-in was correctly listed ad “NewName”.