No exclamation mark

Something wrong with the images?

See attachment: no exclamation mark above my copper mine, although no settler available for it.

Ow, now it's there. There's simply a delay somewhere.
What is causing the delay? I don't assume there is some timer checking every X minutes if there are buildings that need an animated image with exclamation mark.

I've noticed this happening too. There does seem to be a delay of up to around 5 mins before a building notices it's settlerless.
Sometimes clicking the building makes it realise.