Keeps repeating same card, then wont show any

I started using anki 2.1.40 on my computer (mac) and downloaded the latest AnKing medicine deck. The questions work well on the desktop but I wanted to learn cards on my phone so I bought the iOS app (downloaded yesterday from app store on iphone 6s plus 13.3.1)

Synced the decks to anki web and then to mobile.
After an initial interruption where only parts of the media was visible, I synced again one way and everything looks great but after a small amount of questions that I could rate normally I got to one where if I clicked on it seemingly nothing happened, the answer didn’t show and it just stayed there. Every time I clicked bury because I couldn’t rate it, the buttons wouldn’t react. When burying it counted the blue number down to 0 from 80 or so showing one exact card that never responded when tapping in it to reveal the answer. So I buried the same exact card 80+ times even though it technically only exists once in the deck. Then it finally disappeared (when hitting 0) and no card at all was shown. I made sure new card limits are set to 9999 and there are plenty of cards that I haven’t seen in the deck)

when I look at the deck it shows new card but when I click the same deck none show up, just shows a grey screen and 0 0 0. yesterday it was already like that and today it only allowed the same few cards to be reviewed but no new cards. on anki web via iphone safari I can get new cards. (all cards are suspended besides the zanki musculoskeletal deck).
I couldn’t find this issue anywhere and don’t know whats going on… Thanks for any help!

It seems as if the interface has gotten stuck. Please try restarting the app, and let me know if problems persist: Close an app on your iPhone or iPod touch - Apple Support

Thanks so much for the fast response. What you suggested, restarting the app, definitely resolves the issue for some time but after trying it the past few days it does happen regularly again. I tried to figure out if it’s certain cards but not sure I couldn’t see any obvious pattern as to when it “freezes”.
I could always try re-downloading the app and decks unless you have another suggestion. I can definitely live with this, maybe its my oldish iphone 6 that doesn’t run the app optimally

I suspect this is iOS’s web toolkit crashing or running out of memory, which may happen quicker on an older device with constrained resources. Once AnkiMobile 2.0.75 comes out, if it’s still a problem, please let me know, and I’ll see if something can be done about it.