Ubuntu is VERY heavily invested in snaps at a very basic level. I think the recommendation is to not mix snaps and Flatpaks as they may not interact well. As a new Ubuntu user, I'm slowly discovering some of the random problems with snaps.

For example, just the other day, I was trying to configure my fish shell using the html-based fish_configure utility, but it just wouldn't work. Of course, I assumed the problem was with my fish install. After a couple hours fiddling with it, I finally came across a stack exchange comment indicating that the snap version of Firefox simply can't access the /tmp/ directory, which is where fish_config creates its html configuration page. WTF? Also, you can't even install a non-snap version of Firefox via apt because the official apt repository just links back to the snap version! I finally installed an apt-based version of librewolf, but had to get it from a non-Ubuntu repository, and then magically I could access to fish_config html page. That's a pretty long workaround just to view a simple HTML page!

So, if snaps have problems like this just interacting with the base Linux file system, I wouldn't be surprised if random weird behavior cropped up when trying to use Flatpaks.

You do have to add flathub to the discover store, but that's a one time thing and you're good afterwards