Troubleshooting a remote site, the solar panel's output was pulsing at a few Hz and then steady good, then low.
I thought it was a connection/wiring problem until looking at a panel I saw a tiny tree branch shadow with leaves wiggling in the breeze. Then a gust of wind pushed the tree branch with wiggling leaves shadow across the cells. So it's pulsing, random changes in sunlight. Clouds are a piece of cake in comparison, slow and broad coverage. I imagine a flock of birds overhead could cause the same drama short term.
The solar charge controller seemed to get stuck hunting for MPP, which was impossible to find with input power going up and down at a few Hz. I think it needed a few seconds to complete a sweep to find MPP which it did not get. I'm not sure what a better algorithm is because you are wasting energy searching for the highest efficiency on something unstable.
At the time, IDK once the sun moved normal charging would happen, it's just that I was testing at the worst time. At least the firmware did not hang.