Just so you (and devs) know that this is a real issue with real people experiencing it, I have exactly the same problem here with Nextcloud 21 updates. It all goes well until cron jobs run or if I set up a manual occ files:scan job whereupon the server freezes at 100% cpu and ram use and becomes inaccessible.
I take a backup image file before attempting any of this so I was able to revert to 20.08 quite quickly and without fuss.
Would really like to get 21 installed though given that the main benefits are supposed to be a speed up of the web GUI 
I’m using Ubuntu server 20.10 with Apache2 and php7.4 and currently nextcloud 20.08
I joined another thread about this here:
https://help.nextcloud.com/t/freezes-entire-server-computer-caused-by-var-www-nextcloud-cron-php/112359
James