Does it seem as though your phone is discharging while solar charging? Some have said this is a “Reverse Flow” problem and some even suggested adding a blocking diode to prevent reverse flow from happening. This is not the case or solution at all!
If your phone discharges while solar charging, it is most likely the panel is moving under trees or high walls in a canyon etc. Intermittent sun/shadows on the solar panel can cause the phone screen to toggle off and on. If the screen continues to stay on, the phone battery can drain.
About diodes and Reverse Flow:
Bypass Diodes are used on residential panels to bypass panels that get shaded so the others in the sun can continue to generate. Blocking Diodes are then used to protect the solar panel or block current flowing back (Reverse Flow) into the panel in case the solar grid inverter does something weird. Actually, blocking diodes are meant protect the solar panels.
With small USB solar charging it’s a little different. If your device loses power while solar charging it is because the phone screen is not turning off at a short enough interval in combination of not enough sunlight. If you have read that a portable solar charger drained the phone, it is most likely that the panel output was right at the threshold of activating the phone’s internal charger which causes the screen to activate.
Phones will not trickle charge which means, they need a certain level at their port to activate their own internal charging system. Another note is that the phone’s circuit isolates the battery from the input source. The only way you can actually drain the phones' battery directly is to pull it out and put a load on it.
On good note, USB batteries do not have this problem (no screen) and will do better charging in shadows albeit it will take more time. They also add extra weight but if you can’t stop moving (walking) and solar charge, then the USB battery may be a good choice.