The old worlds in the bubble were not "discoverable" originally.
When mapping became a thing there was a bug that allowed the old worlds to be discovered.
This was "fixed" in an update and reverted to how it was before.
Now, whether the ones I've listed above (and many more I've subsequently found) are supposed to have the discovered by tag or not is difficult to tell without having screenshots taken from before the mapping update.
No, you are confusing a few things (and OP badly mistyped his starting post).
1) There were many pre-discovered planets in the bubble at launch.
2) There were many non-discovered planets at launched, and people got first-discovered tags for them.
3) 3.3 introduced a bug that allowed the pre-discovered planets to be discovered
4) This was reverted/fixed, but the 'normal' discoveries were kept.
5) These are now gone too since today.
OP: add 'in the bubble' to your OP otherwise it makes little sense to explorers.