TLDR: Unicode has multiple different ways to normalize certain complicated texts(because Unicode). Time Machine files are made in the most major mode(NFC) and queried in the less common(NFD), which doesn't work. Workaround is to rename backups to pure ASCII. It appear to be first reported on macOS 26.0 and reported reproducible on 26.1.
TLDR: Unicode has multiple different ways to normalize certain complicated texts(because Unicode). Time Machine files are made in the most major mode(NFC) and queried in the less common(NFD), which doesn't work. Workaround is to rename backups to pure ASCII. It appear to be first reported on macOS 26.0 and reported reproducible on 26.1.