Ugh.
With a 13 character minecraft name ---> that just does not fit into a 12 character limit xbox profile name.
And it seems the migration to Microsoft wants to add numbers at the end, e.g.
Minecraft -> 123457890123
Microsoft -> 12345789012
Microsoft -> 12345789012ABC (final name after three characters added at end).
This seems halfbaked.
I'm afraid Mojang Studios can't control the availability of Gamertags provided through Xbox. Their accounting system has also existed for some time, and there are many names that have been taken. I'm afraid you will have to contact Xbox if you want to see if this is possible to change.
Fortunately, this gamertag will not be shown in game for Java Edition!