DRM protected purchases should be able to play in iTunes on a computer which has been authorized to the Apple ID/Account in question. Apple Music for Windows is a pretty poor replacement for iTunes. It is possible this requirement has been overlooked. What happens when you try to play them? Nothing? Or a prompt to authorize the computer which then doesn't fix things? Or something else?
The ability to redownload past purchases depends on content remaining in the store. Rights holders do on occasion remove items from the store for reasons unknown. If you don't see these items in your purchase history check for hidden purchases (see Hide and unhide music, movies, TV shows, audiobooks, and books - Apple Support) or use the Get Info > File tab to check the account they are associated with in case you changed Apple IDs at some point.
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