SOLVED!
Thanks everybody for helping me noodle through this!
It turns out I had a 747 realism mod in my community folder. I removed it, and bang…all the throttles worked correctly again!
I searched for that mod, and sure enough there is an update for it (and for another mod I had installed), so I downloaded the updates and threw them in the community folder, and all is working as it should.
What a relief!!