We a a simple policy that adds certain apps to the Dock. Chrome. Word, Excel, etc. Recently we have noticed that they are now removing themselves from the dock. If they are manually put back they will vanish again.. I am assuming it is Big Sur but it doesn't happen on every machine so hard to troubleshoot.
Our policy is once per user per computer.
I would start by looking for another policy that is manipulating anything in the dock. It could be that you have two policies clashing with each other.
The jamf.log file in /var/log folder of the machine may be able to point you in the right direction if there is a JAMF policy causing the dock changes.