RiccoS

I believe that it's combination of two factors: cloud edge effect, and cooler panels when clouded.
With cloud edge effect, the edge of the cloud seems to act a bit like a lens, focusing energy on the edge. The effect is fleeting, perhaps 10-30 seconds.
Cooler panels have a higher voltage; all diodes are like that, and solar panel cells are diodes.
I believe it's pretty significant. Recently I had a nearly cloudless day, with one cloud lasting perhaps 20 minutes. I took a screen shot of it, but can't find it now. The PV power graph was a lumpy half-sine wave; lumpy because of shading in winter. You could see a bump in PV power before and after the cloud. The bump after the cloud was slightly bigger than the bump before the cloud. The one before the cloud would have been due to cloud edge effect only; the one after the cloud would have been a combination of cloud edge effect and the cooler panels.
Panels get pretty hot in operation, well over 50°C, often over 60°C and higher. Under a cloud, they could cool down to a few degrees above ambient temperature. That could contribute to a little under 10% difference in voltage and hence in power available from the panel while they stay relatively cool.

Lithium cells are well within spec to charge at 0.5C. It is a bit like buying a porshe and then driving it 120km/h. You are not hammering it... well unless you are a grandma and you drive one at 120km/h.
You could charge at 1C ( or even more, like 2-3C for short periods) if eg you have limited hours of sun till the shade hits the panels. This is what lithium does well. The higher the charge rate the more uneven the cells might become due to different internal resistance. You might need more time for balancing if you want to use the full capacity. This is why you have a BMS
So stick to the recommended C ratings, unless you find that you are not getting the full mileage you paid for then you can always up it. Eg. You have sudden loadshedding sheduled and you have important work to finish. Lithium is a very forgiving technology. Your batteries will likely still work fine 15 years from now when we are all running 50KWh packs and Eskom is no more

Both are 0.5C discharge but the actual amperage is different because the capacity is different.

Both are 0.5C discharge but the actual amperage is different because the capacity is different.