hello guys, so im the one who posted the possible fix on the other thread. so my s21 ultra has been freezing up randomly, which ended up restarting. i fully updated my phone but it hasnt been fixed. so far, what has worked for me now, after updating everything was:

settings > battery and device care > battery > background usage limits 

turn off put unused apps to sleep

so far it's really fixed. (hopefully for good) if you're out of options for your phone, why not try this? to other people... try this out please and let me know? thanks!