Same issue so found this post while searching. I have 2 strings of 7 panels each, Longi Hi-Mo 5m 550 W. Spec sheet states "IP68, 3 diodes".
Same behaviour. In morning, sun blocks 1 or 2 panels in each string and I only get ~200 W from each string even with just 1/3 of one panel shaded (EG4...