I have over 60 first discoveries in the Izanagi sector on xbox legacy. NOT ONE of them is mentioned in the codex as me discovering. I turned in the data and wasn't given credit for it. So legacy is literally not updating.
First discovered for you, or the first reported in a region? The only entries that get reported into the Codex are for the first person in the game to discover one type of something in the whole region. The Codex was put into the game in 2018, the majority of reports were made within the first 12 months. If you see an entry in the Codex for something, it has already been reported by someone else.
When you first visit and scan a certain type of object in the Codex for a region you get a 'confirmed' entry added to the Codex and you get a voucher that is worth a bit more than any subsequent version of that object you scan. The Codex only shows you the very first object you scanned in a region.
All Codex entries are registered instantly and don't need any data to be sold.
The Codex itself has many, many issues. I know because I have raised over 25 individual bug reports for it on the Issue Tracker

(I also had 110 first reported entries on PC before Odyssey was released)