Opportunity is missed by most people because it is dressed in overalls and looks like work. - Thomas Edison
Peeling it cold will leave a duller finish.Dan there is a big difference between running a shirt through a drier and its heated air versus applying heat directly under pressure.
It is possible that the inks used are just very tacky. The other question is: How much pressure are you using to apply your vinyl?I have found all different ink types can stick/transfer at least a little, and that likelyhood increases as the print gets thicker. The big variable is ink chemistry and pressure from the heat press.
Dan, do you mean ThermoFlex? I doubt the numbers were under cured, it will happen to us sometimes and I know our numbers are cured. Like Frog said, use a sheet of kraft paper. We use that no matter what to keep the ink off the teflon cover.