It is related to MCL-16570 as it is about a purchase being shown as a demo version
However, the different detail is that the Microsoft account and the Mojang account uses the same email
For the record, here's the fix from the linked ticket "Minecraft: Java Edition (purchase after December 1, 2020), sign in using your Microsoft Account."
But, as far as I can see, there is no way to sign in with a Microsoft Account when the Mojang account has the same email when not logged, it's only an option from the settings, which requires to be logged in, and that sounds like a legitimate usability issue (at least until the Migration process kicks in)
Your title says it can't open "snapshot", but what snapshot? 😉