Domains manually added to the whitelist won't be blocked.
Actual Behaviour:
Since i updated PiHole from 4 to 5 all domains on whitelist (added before the update and after) are still blocked by PiHole although they appear on the whitelist, after pushig the "Whitelist" button in the Query Log.
Did you move your clients to another groups as the "default" one? If you press whitelist in the query log, those domains will be added to the default group. If all your clients are in another group, whitelist won't work for them.