A couple of Wombats stop running between early afternoon and sunset

Two of the initial installation are stopping around early afternoon and coming back at about sunset.

This started 3 days ago, before that they were fine.

It seems the ESP32 is not getting powered up because none of the missing messages are coming through after it wakes up.

I don't think the modem is the problem because it can handle up to 4.6v and we don't seem to be getting over 4.2v.

Could it be the 3.3v regulator, or components around it?