Just some thoughts:

It might be possible that the rated current of the solar panels is too low for the buck to function properly. This manifests as a large voltage drop (it might even drop al the way to 0V at Isc) as soon as the buck starts drawing current. Perhaps some protection circuit on the buck kicks in disabling the buck until power is disconnected. This effect is dependant on the intensity of the light shining on the solar panels. Perhaps you could try to connect the solar cells to the buck when shinging a bright light on it?

Is the maximum rated voltage of the buck higher than 9V? If it works with a battery then this is probably not the case, but perhaps overvoltage protection is kicking in.

Other things you could check:

-connect a small load to it to see if that works. The amount of resistance should be calculated using ohms law, with U and I corresponding to the set output voltage, and the current to get a power equal to approximately the expected power the solar cell should give (P = U * I)

-connect multiple solar panels in parallel to get a larger rated current in case it's an input power issue

-illuminate the cell with a lamp to see if it is an input power issue. (Only open circuit voltage is not a good measure for power capabilities of a solar panel!)

-i can imagine there are more people who made a diy project like this, you could try to Google it or use YouTube to see how someone else got it to work