I noticed the yellow head icon over a mine indicating a vacant workers slot. So I selected the mine to add the missing worker. I pressed the plus sign and a worker appeared, only to imediately disapear again. I then noticed that the main allocation selection of how many workers to use was set to 7 and not 8. I then tried to press the plus sign to increase the numer of available work slots to 8, but noticed that the plus sign had been made unavailable.
Firstly, I don’t think I have changed the numer of work slots from 8 to 7. Secondly, if only 7 slots are available, then the yellow missing worker icon should not appear, nor should the plus sign on the worker slot be active to indicate a posibility to add a worker? Thirdly, should the player not always be in charge of changing the number of available worker slots?
Bug?
PS. I have no idea of if it is related of course, but I made the observation that the merchant-present-icon-lit-when-there-is-no-merchant-present bug was in effect at the time of this behaviour happening.
EDIT: This issue was resolved by saving and reloading the game. After reload themine has 8 active worker slots, all filled.