Consider upgrading to macOS 12.4 (current), though I believe at some point, you told the Mac to learn BUENA VISTA and added it to your spelling dictionary. Along comes Apple Mail, where you may have Spelling and Grammar > Correct Spelling Automatically enabled, and it sees Buena Vista typed and then auto-corrects it to upper case. Turn this automatic spelling correction off, and enter Buena Vista again to see if it remains in its expected mixed case.
My Apple Mail in macOS 12.4 does not upper-case Buena Vista, with or without Correct Spelling Automatically enabled, and there is no entry in my /Users/USER/Library/Spelling/LocalDictionary for Buena or Vista.