Assuming it was tested in brilliant direct sunlight, I suspect it is due to the “iSmart” technology embedded into the USB port outlets. It’s designed to deliver the optimal charging current to connected devices with batteries (i.e. a mobile). It will try to “read” the connected devices requirements, and will undoubtedly have various protections (anti surge etc) which may be causing the issue.
Although the numbers appear to suggest it is possible, in a real life situation you may find yourself asking a bit too much of the solar cell.
If I was designing this from scratch I would take the native voltage from the solar cell (very probably 12v) to storage cell, then a circuit to allow the 18650’s to be charged from that.
In your case that means chopping the USB ports off your (rather beautiful)solar panel, and cracking out the soldering iron, and binning the charger etc…
Even I would be reluctant to do that, and I think given those components I would just add a USB power bank. Charge that from solar cell, then run the charger from that. They are available at 20,000mah up to 50,000, and will deliver 2.1A (so you won’t be “fast” charging 4 batteries at once).
You retain the ability to use the solar to charge your phone, and you can charge your 18650s using the charger from the power bank (charged from a wall outlet), whilst out and about, until the zombie uprising occurs.
Source: I am an electronics engineer lol