I've never noticed that, but I just tried and confirmed.
My Guess is that during EN pin Shutdown, the PMIC reverts to it's default/basic configuration.
The results would suggest that the Minimum Voltage or Current isn't being met with a Computer USB port, but can be met with AC/DC charger, which makes sense.
But I seem to recall confirming that the user defined PMIC settings were obeyed after EN Pin shutdown.... at least back in 0.9.0 firmware days. I could be remembering incorrectly.
System Power Manager can now change all your PMIC settings behind the scenes now, which significantly hurts Solar Powered Borons. I've paused my work with Solar Boron's for now until that's addressed. Most of my long term testing is invalid now.
Of course, a 12V Panel charging a SLA is still a good option if you have the space.