The PLB gives out a digital alert signal at 406Mhz to a Low Earth Orbiting LEO satellite. I suspect that if the signal to noise ratio for the reception of GPS signals at the earths surface, which I think limits at around -140dB, has been degraded by solar activity to point when PRN lock is unavailable then the 406MHz signal to the LOE satellite will also be degraded also to point that it will also fail. In general a satellite has a harder time detecting a signal broadcast from the earths surface rather than the other way around. This is why they are Low Earth Orbiting.

Because the GPS system uses PRN or Gold code techniques, which lowers the effective noise floor, this allows the GPS receiver to decode the signal from the NAVSTAR satellite, which would have otherwise been impossible. Higher quality GPS systems have superior antenna gains allowing more robust GPS PRN signal lock.

Many PLBs have a GPS integrated into the design (the integrated Antenna which will have poorer performance than dedicated antennas). This integrated GPS would also be affected by the same solar event which will either cause no or poor position computation depending on the amount of ionosphere noise. If the PLB digital alert was picked up by the satellite (again depending on the level of ionospheric noise) then the PLB would have to rely on the traditional 121.5 MHz homing signal for the rescuers to home in on. Therefore if your Garmin GPS is not getting any PRN (Satellite) channels at all then I would suspect that the PBL will not function either in that the 406MHz digital alert will also fail to communicate with the LOE PBL alert Satellite.

If the Aurora Borealis was visible at tropical latitudes especially during the daytime/early evening then I think you can say bye bye for good to all the earth orbiting navigation and communication satellites together with all the worlds electricity grids (would take many months to get working again). It would be a pretty spectacular fireworks display though.

So to summarize, if your GPS isn't working because of a solar event then your PLB will probably not work either.