The theory I’ve always been led to believe (from Romanians nonetheless) is that when Aurelian vacated Dacia, he only took the administrative people with him. The actual Romans citizens still living there chose to stay back and continue living in the vacated province for centuries. And they continued to have commerce with Rome via Moesia and Pannonia.
It’s an interesting theory because after Aurelian vacated, Dacia disappears from history for about a thousand years. In a way, it went through a similar, yet even longer transition from Roman Province, Dark Age Period, to Middle Age Kingdom as England did.
I lived in Romania for several months and there is no denying that modern Romanians are of Latin descent, just based on looks alone. The only Romance language I speak is Castillian, and after a week or so I could probably understand half of what was said to me.
That's likely, moving everyone would have been a hell of a big job. The ancient blobs of concreted coins that used to proliferate on eBay were largely sourced in the former Warsaw Pact countries and the Balkans. These were generally attributed to burial for safekeeping by households under attack or areas under invasion during late-antiquity. After the danger passed, the owners apparently never had the opportunity to retrieve them for whatever reason (death, exile, captivity).