My guess is either a faulty Garo unit or a regular supply voltage fluctuation that is too short to be picked up by the sampling the car charger is doing hence you can't see it on the app.
Leaving a decent multi-meter connected on min and then max might prove the point, failing that a voltage data logger could be hired and connected for a few days.
Even if you prove that, getting the DNO to do something about it can take effort and endurance.
To be honest I don't highly rate the methodology used by the Garo Pen protection devices as they aren't guaranteed to detect an open PEN conductor in all circumstances (subject to what loads your neighbours are pulling at the time) and they can be triggered by supply issues (as you may be seeing).
If it were me I'd be seriously considering banging an earth rod in and changing the earthing arrangements in the garage, removing the need for the PEN protection device completely.
I don't want to cause additional concern but a couple of unrelated things jump out at me too:
-you may have a PV system on a shared RCD in the house CU. This is a no-no as there's no point the RCD tripping to save your life if the PV system continues to deliver power until it notices it should shut down.
-the RCD topology isn't ideal - I think there's three 30ma type A RCD's in series. A Type B would normally be fitted for an EV charger.
