Minecraft account migration not working or failing? Here's the official word

Minecraft players are reporting that account migration is not working or failing. Here is what Mojang has to say on the matter.

Postmigration issues - Can't log in with my Mojang account email address. : r/Minecraft

Minecraft account migration, something went wrong : r/Minecraft

Minecraft is Deleting your Account Soon!

Java Edition 1.9 – Minecraft Wiki

It says I don't own Minecraft after completing migration. what happened? : r/Minecraft

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Guide] How to fix Failed to login: Null after migrating your account

Custom – Minecraft Wiki