I should note, that for real world solar panels, the power supplied by the panel is dependent on the solar photons received. For a slow change in power delivery some circuits have some strange behaviour. I’ve had a Mayfly with an LTE module, and a 2Ahr battery, and at sunset it started resetting. It continued resetting for about 30minutes until the sun’s power was completely gone. I had let it run and this repeated every night for a week. I changed out for a 4Ahr battery and the resetting stopped. The 2Ahr battery was being fully charged every day, and when the sun was down it ran OK overnight – just in the twilight zone it had a problem. Was OK with sunrise.
I was monitoring the LiIon battery voltage with both the Vbat ADC and also externalV with 100K-100K divider. The Vbat ADC was showing a funny dip in voltage before it reset – but I never investigated further – just made sure I used the 4Ahr battery. I am doing some characterization work how to measure a battery that is below 3.7V – but still ongoing.