Removing Teams app from the left rail
If the app setup policy assigned to the user is changed to block user app pinning, Teams removes any apps pinned to the app bar. If the policy is then changed to allow user app pinning, users must re-pin their previously pinned apps.
But another case is If the policy is allowed user pinning and then update the policy by pinning additional app or removing app from admin centre will also remove the user pinned apps.
Overall If admin updates app setup policy, all the user pinned apps will get removed in all cases.