I think some later models maybe came with the sheet from the factory to try to eliminate the problem.
The particular example from the mid to late 90's that I remember as being the worst (of the models I had or observed) was the 16955 Santa Fe flat with Challenger (example here). The Challenger had treads and every example I have ever seen had the treads pattern somewhat "melted" into the surface of the flat (or at least made a permanent mark). Some of the worst ones (where there was never anything put between the treads and the flat body) look like the treads are actually glued down to the flatcar after all of these years.
I don't have all the cars you show above, but I did have a few of them. The top yellow flat with red tractor I did have, and I think mine came with the plastic from the factory (or it's possible I bought it after a dealer had added it, but I'm pretty sure I don't have tire melt on that car). The 16955 I think maybe didn't ship with the plastic sheet, though some may have had it added by dealers or first owners once the problem was identified. I (IMO) think this may be one of the hardest (maybe impossible) pieces from the mid 90's to find in true "mint" condition since every one I have ever seen had some level of melting.
-Dave