When merging records, users were used to seeing the result of the merge straightaway. If something prevented the merge, a message describing why would show in the user interface.
Now as a result of a change in August 2020 (online), users see this message.
And if the merge fails, the user hasd to look in Systems Jobs and search for failed jobs with the name Merge.
There is a way to revert to the old behaviour. Log a support ticket with Microsoft and ask to switch the merge behaviour back to synchronous. When I did this, I was sent a solution (FCB.Editor) to install and then within the Configuration section of the solution, told to add a feature and setting to change the behaviour.
The solution has to be applied to every organization where you want to change the behaviour. The feature is FCB.EnableAsyncMergeAPI and the setting false.
I can confirm that this worked for me. Afterwards the solution can be deleted from the organization (but keep the solution file in case you need to use it in new orgs that you create).