We recently experienced the same condition. Our M300 is less than a month old and has only been flown a few times. Last week, while performing a mapping mission, our M300 went from tracking 18 satellites to briefly tracking 23 satellites and then tracking zero satellites. The M300 went into Atti mode and we manually returned the M300 to the launch location. We are running the latest firmware on our smart controller and UAV.
After speaking with our DJI dealer, it was recommended we ground the M300. The dealer shipped us a replacement unit.
The following day, we contacted DJI to request a return shipping label to return the M300 for repair. As part of DJI's process due to number of satellites = 0, we contacted DJI's support phone number. The technician from DJI recommended powering the M300. Upon power-up, the M300 was tracking satellites again.
We cautiously performed our mapping mission and were successful mapping over 650 acres without GNSS satellite tracking issues.
We are still planning to return the unit to DJI for review. I did hear from our dealer that GPS loss is the leading issue reported on the M300. I will plan to update this post if I receive any resolution from DJI.