'50s Ice Trays....
Yes, they did start sticking after a time.
The reason was that they came factory-coated with a very light wax finish, to aid release.
Using them degraded the coating, and washing them pretty much eliminated it altogether.
I've never tried it, but it seems to me that rubbing vintage trays with a bit of melted beeswax or paraffin wax could restore their non-stick quality.