To try to fix Photos app Reset and Repair Photos app at Settings > Apps > Apps & Features > Photos app > Advanced Options. Your pictures will be safe in the User Pictures folder.
Wanted to contribute as gratitude, by providing some feedback for others that this (quoted above) fixed my issue with Importing media from my iPhone running IOS 13.3. Prior to applying this fix, Microsoft Photos would stall during the scanning phase, specifically after showing about 399 detected photos, then the animation would stop and the app would not respond to any UI interactions.
If resetting doesn't work, I would recommend folks here to try to change this one setting that fixed a different problem I had importing individual media (that is large in size) -- Go to the iPhone settings for your Photos app, and then scroll to the bottom section labeled "Transfer To Mac or PC". Switch this to "Keep Originals", and this should skip the process of checking the files for compatibility, which has fixed my issues with importing some large GB's sized video files.
Good luck!