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That's how the system works. Depending on your scientist's skill and the digsite's difficulty, they'll keep rolling to gain progress in the excavation. If the digsite is out of their league it can take many "rounds" to make progress.
You should be able to play using 2.2 saves with 2.3, though due to the present changes, stuff might be a bit wonky. Especially with sector generation (that system underwent a rework in 2.3) and some balance, so your economy might go out of whack.
That said, saves should work on principle.
If you want to enjoy the new content however, you will need to create a new galaxy to have it populated with it.
