I’ve had the same problem with multiple sets of wheel. I clean them with Simple Green to get most of the crud off, then spray some Brake Clean on a rag and scrubbed both sides of the wheel lip.
I know you said you confirmed that they are not leaking from the bead locks, but it’s a good idea to change this seals. I changed them last year and they were already cracked and dry rotted again at the start of this season. Also a good idea to run tap & die kit through the wheels and bead lock screws. Hopefully that helps.