I am alone to tackle these type things but usually sort things out.....usually.
Do you have another ( as in unused) on- off switch? I might try to replace the non dimmer light switch with a new one. Easy to test and low cost option. Could be something faulty in there.
Like I said- I ain't no pro... but no harm trying these.
Also are the wires connected to the nuts or in the holes in the back of the switch. I have seen when ( in cold days on exterior walls it happens more) that an undersized wire is put into the friction fit hole then the contact gets slightly loose. Move the wire to the nut and voila.
I’m with Ken on this replace the 3 way dimmer with a 3 way switch and see what happens. This sounds more electronic than actual electric issue. Dimmer do funny things sometimes. By any chance have you replaced the lamps with LED lamps?
