You're right Barry.
I tried it again, and this time the correct forms database was picked up appropriately.
When I first installed Domino 9.0.1, I had uninstalled Domino 8.5 beforehand, and therefore the Forms85.nsf file was gone.
Then I restored it, restarted the http task, used "tell inotes flushforms", and I THINK I even restarted Domino, to no avail.
But I see now both forms databases are working well together.
Also, thanks Brandon for pointing to inotes_wa_formsfiles, this may come in handy.