This sounds like something possibly unique to the Note 20. iFly works fine in Landscape (horizontal) mode on all of my Android devices, including 2 Samsung tablets and a Samsung S9+ phone.
iFly is not designed to operate in split-screen mode. I'd recommend you avoid trying to use that capability with iFly, to remove one variable from the situation.
Make sure you have the phone oriented horizontally before you launch iFly. If you launch the app while in Portrat (vertical) orientation, and then change to Landscape (horizontal) while it's launching, that can cause problems.
So: Starting in full screen mode, in Landscape orientation...when you then try to launch iFly, what exactly happens? (Does it not launch at all? Does it start to launch and show the "iFly GPS" splash screen, but then exit back out? Does it get to the map screen, but have display issues? Some other behavior?)
The iFly folks are at Sun'n'Fun in Florida right now, so they're not monitoring the forums. They should be back online here in a few days, and you can wait and see if they have any suggestions.
Alternately, you could try submitting a bug report (which the iFly folks may ask you to do anyway): Go launch iFly in a way that causes the display issue you're seeing. If the app exits on its own, fine; if it doesn't, then do what you need to do to close the app (by force, if necessary).
Then, in Portrait mode, re-launch the app, and choose "Menu > About > Send Bug Report", choose "Use Default", and then pick your email app when prompted (Gmail, or whatever you use). iFly will populate a new email message with a bunch of information; you can write a short explanation of the problem you're reporting at the top (right before where it says, "Current Log:"), and then send it on its way.