Let's just check people are not getting confused.
If a system is pre-discovered then there will be no discovered by tag.
If a system can be discovered then there will be a first discovered by tag.
If you are entering a system and scanning for the first time, you do get to "discover" it, but of course you don't get a tag. Only the first person gets that honour.
A lot of systems in the bubble are pre-discovered, so make sure you are not looking at one of those.
If indeed, non-pre-discovered systems are missing their first discovered tags, raise a bug report.
What OP says, I've checked a dozen or so small systems around Sol that definitely had tags, and they are all gone. Its the same old bug again as when 3.3 launched.