Solar voltage waivering
The PV needs a minimum current to start the converter out in the sun,  Indoors is like &lt;1% of the current and insufficient to start it. It may need 5% of max to get started.  

This has to do with the frequency range of the converter and the load impedance at a low frequency of the inductor.  Once current source rises, it should be able to sustain the voltage until you apply a load that exceeds the supply. This is what a MPPT regulator  will do match the load impedance between load and source to enable 80% of open circuit voltage.  A simple LDO wont be able to capture the full power like a motor running a low RPM.  You need a converter than runs at 80% full RPM or voltage in this case to convert to 4.2V.  

Try measuring the following.


under a 300W lamp


Voc (open cct.)
Isc ( short circuit &lt;1A range)
connect battery without regulator and measure charge current

then connect a large cap across PV , check voltage   (1000uF)
then attach LDO and test.


then report results


  Or simply add a large cap to PV and verify LDO connections.