The ships existence wasn’t broadcasted or well known. Only two of her class were built and both had an extremely classified propulsion system that wasn’t talked about outside of Admiralty, Section 31, and specific Enterprise crew.
spoilerAlso Discovery wasn’t marked down as missing. She was marked down as destroyed. There’s no real reason for Lower Decks to ever mention Discovery in any fashion without it being a super reach. It’d have to be something like just adding ‘Discovery’ to a list of ships destroyed during a specific period.