In fact, I think the whole voxl-mavlink-server scheme to set the system time from the SYSTEM_TIME mavlink message is fundamentally flawed for the case when PX4 is run on SLPI. The SYSTEM_TIME message emitted from voxl-px4 uses the system clock of the apps processor as it's time basis. So essentially, it either

never changes the system clock if the offset between SLPI monotonic clock and apps monotonic clock is less than ten seconds continually sets the clock to either the future or the past by the offset of the monotonic clocks in the case where the offset is greater than 10 seconds

It seems like this system should really be based on GNSS clock time instead of the SYSTEM_TIME message for this setup.