I have free settlers, but they don't working

[Torq74]

I have 0 free settlers. I built a farm, naturally nobady working there. I upgrade this farm. I have free settlers, but nobody working the level 2 farm.
see the attachment.

[Torq74]

After 2 hours, the settler begin working. :D

[Dilbe]

This is a known bug that has been in the game for at least a year. The building does work, it just does not show there is a settler in it if you upgrade the building while you have no settlers available.