Enphase IQ7+ minimum Vmp is 27V, 

when one sub-group is shaded, the voltage become 2/3 x 33 = 22V. 

so, IQ7+ can not track the maximum point, it is why IQ7+ can not handle correctly

First, I do not know, it has been caused by an Inverter or a Panel,

but, I am suspecting that the problem is happening by The Inverter.

Because I have verified with MPPT charger with same panel, and similar condition,

and, the power generating is +1/3 and shade two cells, and +2/3 when shade one cell.

Everybody know the shadow can reduce the power from Solar Panel,

there are 3~4 strings in a panel, if shading a string, the other strings can operated normally.

What will happen there is very small shadow 1-2 cells in a 60 cells panel ?

Yes, it can reduce the production to 30~60%,

Look at the detail production yield at my house,

Sunlight is coming from Right-Top side and, Dommer at Right Hand Side is making a shadow on Panel #6,

#1, at 09:19AM, half of two cells (right-top side, two cells in horisontal, not a panel) are shade, power production is just 10% via the other panel.

#2, at 10:23AM, very thin shade on a cell of panel (around 10~20% on right-top side of cell, not panel), production is 80% via the other panel.

#3, at 10:29AM, no shades, same power gain via the other panel.

In the solar panel test with MPPT charger,  the minimum power is aroudn 60% in #1, 90% in #2

It makes me suspect There is a critical problem in Enphase IQ inverter.

Note. Enphase IQ 7+, LG350N1C-V5