Your observations were discussed at our coffee-shop klatch this morning
& a new possible theory came out. A local EAAer had a factory reman
Continental C-90-12F that would sag immediately post-takeoff. He
wrestled with the problem for about 300 hrs before he tore it down.

He found that there was insufficient ring gap and that the cylinder
bores had become glazed such that they looked like mirrors. He rehoned
them and reassembled & it fixed the problem completely.

I too find it hard to believe, but it might at least be worth a look
into the cylinders next time you have the plugs out.