I'm fairly certain this isn't new in macOS 15, I could have sworn I've seen this on 13 + 14
The Mac mini in question has macOS 11 through macOS 15 installed on separate APFS volumes. I just tried booting into 14 and 13.
I'm not sure why I never noticed this before on Sonoma. Perhaps it was added in a later minor update, but I have no idea.
I can't reproduce on Ventura.