Well, for me this is just obvious every time, set a fleet order, wait... I guess, the only factor, which can change in here is the ship itself, so I am including the ship blueprint file just in case you need it.
{F12699}
I've only did the ship today, and found this bug out today. I was not commonly using fleet mining before this moment, and can't tell if I've experienced this with smaller ships/earlier versions.
I do not include any techinical specific information or logs, because I experience no crash/lag. I think the issue is either in the coding, or me doing something wrong with the ship.