GPS status dropped to 1 for 1 - 2 s, would like to know possible reason

I am using simpleRTK2b as my primary GPS, occasionally its status will drop to 1, which shows GPS: No fix for a very short time, then it recovered and resume mission. Not crash probably thanks to the secondary GPS, but still, its better to be resolved.

Note this only happens in auto mode.

Here’s the graph:

the log: