@Luba Local songs you own (purchased or ripped) should never be touched or replaced by Music app unless you intentionally delete them.
The problem in your case, if I understand correctly, is that your original ALAC files are no longer available because the Mac Pro doesn't work anymore.
All your working devices using the same Apple Music account used by your Mac Pro rely only on music available on Apple's servers which were previously matched from your Mac Pro.
Apple provides AAC version of that tracks because at that time this was the way it worked. Now with Apple Music lossless, new matched songs are provided in ALAC.
Basically you don't have anymore your original ALAC files, unless you recover your ripped files from the Mac Pro. There's no reason for ripping in AIFF. You can keep ripping in ALAC, in my opinion. In the beginning I ripped in AIFF, then I switched to ALAC.
I tried to delete and rematch an album and Apple still gave me the "old" AAC. Only music that I never matched before is provided after matching in ALAC.
Currently it seems that it doesn't help to rematch the music that was matched before the introduction of Apple lossless. I hope Apple will eventually replace AAC versions on their servers with ALAC counterpart.