Oddly enough, in the iPadOS 15.7.1 at least if I say "Hey siri, set a timer for 20 minutes" it will say "20 minutes, starting now". Then if I say "Hey siri, set a timer for 5 minutes" it says "there's already a 20 minute timer. Replace it?"

If I say "set a timer for 20 minutes called A" it just ignores the "called A" part.