I've encountered this same issue and while I don't have a good solution, I do have a solution that will work. Adding "albums" to an existing "artist" folder works great but when I need to add a new "artist" folder at the top level of my library, it does not get picked up.

The solution I've found is to close the application, delete the strawberry profile data, relaunch, and reconfigure the application. Historically I've deleted ~/.var/app/org.strawberrymusicplayer.strawberry completely but you can probably cut a few steps out and only delete the "data' subdirectory.

This isn't a great solution but it does work. One of these days I may try to figure out if I can find logs of the scanning process (if they even exist) but for now this gets the job done.

I'm currently on Linux Mint 21.1 but I've had it happen on several earlier version of Mint as well.
Interestingly enough, I had this same issue when I was using Clementine earlier in the year before switching over to Strawberry.
My library is held on an external disk and originally I thought it had something to do with that or some sort of weird file system permissions but no matter what I did, the issue persisted. The only thing I could figure out was to blow away the library database and start fresh.
I have a few ideas of reorganizing the library structure so I don't have over 1,400 directories in the top level but that will take time I don't feel like spending.
Is there any way to log the directory scanning process so I can try to chase this down?

I've encountered this same issue and while I don't have a good solution, I do have a solution that will work. Adding "albums" to an existing "artist" folder works great but when I need to add a new "artist" folder at the top level of my library, it does not get picked up.

The solution I've found is to close the application, delete the strawberry profile data, relaunch, and reconfigure the application. Historically I've deleted ~/.var/app/org.strawberrymusicplayer.strawberry completely but you can probably cut a few steps out and only delete the "data' subdirectory.

This isn't a great solution but it does work. One of these days I may try to figure out if I can find logs of the scanning process (if they even exist) but for now this gets the job done.