I have Re read the op's post and I keep getting hung up on the "at TDC" and I feel like I am missing his point somehow. I have been a proffessional mechanic for over 30 years and have not heard of compression related to top dead center . It may be me or the op is describing something in terms I'm not understanding.
Compression ends at TDC and therefore is lost at TDC , it does not stay compressed long . I do know many people have had concerns about TDC over centering or that loose rocking feeling at TDC , which is normal.
My questions are , does it still start and run?
Do you feel any compression build up at all when flipping the prop?
Please build me a better picture of what is happening when you are noticing the loss of compression at TDC .
You have me very curious.