What is the ruling on one who grinds grains, if something from that flies into their throat as a result, while they are fasting?

The fast of one to whom flour dust or the like flies is valid and not invalidated because it is not by their choice. The fundamental principle is that things that break the fast do not invalidate it except under three conditions: knowledge, remembrance, and choice. So, whoever does something that breaks the fast ignorantly of the ruling or the time, or forgetfully, or under compulsion, their fast is valid.