I don't understand what you're saying here; are you talking about captured workers or, less likely, resources "captured" by culture.Is it possible that when those were first captured they were captured by forts while outside the any city workable radius then once the city expanded its radius they came back around to update the plot?
That might be the case; I didn't try to build new workers after the update but I did try to manually move them for some turns and then turning on their automation again. When I did, they went straight to the first resource that was worked by a city to build a fort; when the fort was done they built the proper improvement, when the proper improvement was done they started on a new fort; I think you catch my meaning.I suspect that between that and the game having a mid-game update recently to the new AI might have caused some worker scrambling to react.
At one moment I had 8 workers building forts at the same time, when there were cities connected by culture that didn't have a single improvement and they didn't even consider to upgrade trail paths and mud paths into roads.