Been that way for a while.

Seen 1 for sure but never a decimal in front like that

Are you talking about mempool.space or your own instance?

True, this is from space. Usually I use my own instance

It started like 6 months ago. A bunch of sdbitcoiners were making sub 1 sat/vbyte transactions but not all miners were accepting them. Eventually core and mempool . space updated to be able to track pending txns better.