Hello there,
just want to report that when I cast a spell that applies velocity to blocks (like Blast or tornado for example) and hit blocks that are in a worldguard region with the flag "block-place" set to deny, blocks from in the explosion area are being dropped.
This doesn't happen at all when I'm in a region without the flag.

Yes, it's a known issue. Something weird that WorldGuard. Their response is to not use that flag, since just making a region is enough to prevent players (non-members) building.

So if you remove that flag it should still prevent building, but the bug will go away.

I've removed the flag but players still seem to be able to place blocks, here's a screenshot of the flags
If I set passthrough to deny then players won't be able to cast spells in the area.

Yeah, passthrough means people can build!

if you want them to be able to use destructive/building spells but not destroy/build by hand, remove passthrough as well but add "allowed-spells *".

Hoping this helped, unfortunately there's not much I can do about the core WorldGuard issue, and their team does not seem interested in fixings it.