Yes, poplar moves a ton with moisture changes. Even nicely kiln-dried poplar from out east shrinks noticeably when I store it here in NM for a few months. I tend to avoid poplar for things where it’s going to see big moisture changes, but if you can only get some of the veneers you need in poplar, that’s a tougher nut to crack. Shellac might help, but my guess is that it still wouldn’t solve the problem, since it’ll still let water through, just more slowly.

That said, you got great results. Good job flattening it out with the iron and then moving to a scraper to flatten the surface.

May you have the day you deserve!