I have been doing alot of work with the compatibility of chemicals, and I can only offer a generalities with Polyurethane, as many manufacturers add special ingredients to provide unique properties for customers. These special ingredients often make the compatibility between these two products even more incompatible than the pure materials.
The information I have on Millable Polyurethane and Formaldehyde (full strength) is that is is 'sometimes OK for static seals, and not OK for dynamic seals'. The compatibility 'ratings' decrease when the temperature is increased above room temperature.
I hope this helps,