I made the small side table project in maple and ended up using clear shellac with a couple light coats of polyurethane on top (see related post: https://woodworkingmasterclasses.com/discussions/topic/how-to-finish-this-maple-table/#post-681690). The table top was looking great after 3 or 4 coats of shellac, so I put a coat of poly on the underside — and later when I flipped the tabletop over I noticed a couple scratches on the topside. The scratches were in the wood itself, so I sanded down just that part to the bare wood to get the scratches out, then reapplied shellac to that area.
Then I realized I had just made a big white blotch. So I figured I could lightly sand the surface and reapply shellac, which I did a couple times. This “worked” to get the blotch to mostly match the rest. But… in the process the tabletop formed a cup on the bottom (so there’s a “hill” on the top). I figured this was due to having put more finish on one side vs the other(?), so, having already put a coat of poly on the underside, I decided that maybe adding another coat of poly to the underside would help. It didn’t of course.
So now I’m wondering if I should just sand the whole thing down to wood and let it sit for a few days to see if it corrects itself? I haven’t tried attaching the top yet, so I’m not sure if the screws/turnbuttons would flatten the tabletop.
Poly seals more than shellac. To me, it sounds like the top absorbed more moisture than the bottom, thus the belly. I don’t know if you can get all of the poly off without damaging your table, but you might try that. Then re-sand or scrape the surfaces and let them stabilize for a couple of days. After things reacclimate, you might have to plane or scrape the top flat again. I. understand your thinking on using the poly on the bottom, but I believe that you need to use the same type of finish on the top and bottom. Good luck