I bought Minecraft on February 1st, 2020, and still have the email receipt. Today while opening it up on my new computer, however, it is saying, "Play Demo," where it would normally say, "Play". Another person (Aaron Burke) reported the same issue, and I feel there may be a problem going on here. I would love to play Minecraft Java Edition, however I can't until this gets resolved.
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However, this issue is Invalid.
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So this turned out to be my Mojang account not being connected to my Microsoft account (or something like that) since it has been a while since I have used either!