Edit: Resolved.  Not a bug.  Perhaps bad UI design, but not a bug.

Original Post:

Mojang account is migrated to Microsoft account.

• CAN log in fine to both microsoft.com and minecraft.net using Microsoft Account email and password (followed by successful 2FA authentication by phone app).
• CANNOT log in (using same Microsoft Account email and password) in the Minecraft Launcher.
• The same password is copied from a Password Manager each time.  There is no chance it is being entered wrong.
• The launcher was just downloaded today from https://www.minecraft.net/en-us/download after logging into that minecraft.net with my Microsoft as stated above.
• The launcher version is the Debian based installer (https://launcher.mojang.com/download/Minecraft.deb).  This is the same version currently in the Linux Mint repo (package name minecraft-launcher, version 928).

Here is something to note.  Both my old Mojang account and my Microsoft account use my SAME EMAIL ADDRESS (it is a GMail address).

The following behavior is of note:

• Whenever I enter my email and MICROSOFT password, I get one error: "Invalid email or password" (Screenshot 1).
• Whenever I enter my email and OLD MOJANG password, I get a different error: "Invalid email address or password. If you already migrated your account, please go back and sign in with Microsoft." (Screenshot 2).

This tells me something serverside is receiving my login submission and is aware of which password I sent.

SUMMARY:  I am submitting my valid Microsoft Account credentials through the Launcher, yet the error states it is invalid.  I cannot log in.  I cannot play Minecraft.