I wonder if that one Mono in low light/shoulder hours would be the same as having a shaded panel in the string during that time.
If I understood JiminDenver, the Mono would act the same as the Poly midday, but lose more output than the Poly in the shoulder hours. So having the Mono in a series string of Poly panels would not be ideal.
That would mean the OP has the one Poly shaded midday and then just as that clears up as the sun moves around, the Mono starts acting like it is shaded.
A possible test for that might be during when all panels are clear of shade in a shoulder time, to put the blanket over the Mono and take string readings, and then quickly put the blanket over a Poly and take string readings. Losing the Poly should hurt more if this theory is correct.