Fleet agents go offline then return healty

Hi, i have 3 agents other than the fleet server.
this morning all 4 were healty, now 3 of them are labelled as offline (included the fleet server) and one of them is still healty. How is this possible? and why sometimes they turn healty again (for quite a long time, e.g. 3/4 hours) with me doing nothing at all?

they seem to turn healty / offline totally random.