Hope you're doing well. It does sound like an update on the app will be required for it to work smoothly on the latest Android version and latest Samsung devices. If you haven't already, it may be ideal for you to let the app developer know by leaving a review on this via the app store you downloaded the app from as all app developers are always looking to improve, and appreciate all feedback and reviews.
Same problem with the note20Ultra. You would hope that during a Samsung-Niantic discount event(10% off via Galaxy store) the flagship device would be tested before launch.
I have contacted Niantic also, hope their developers will get it fixed before the event is over.
Just got my Note 20 Ultra today and discovered I have the same issue. Sometimes it locks up almost immediately, other times it'll last a bit longer. It took me many attempts just to submit a bug report to Niantic. Not impressed that this looks like a repeat of the same issue with the S20 phones either... wonder what causes it?