If you can’t see the tractor’s mirrors, they can’t see you back there, either.
Back off till at least both mirrors are in sight.
The situational awareness of truckers is probably quite good. I think they generally know if somebody has come up behind them and then not passed. Also, modern trucks probably have rear camera assist - much like many RV's. And if you are in their mirrors, of course they know you are hanging back there, and might prefer you didn't. At any rate the issue of road debris is enough reason, IMHO. During the years that I was riding motorcycles that was the primary reason I avoided the temptation to draft.