Solar panel voltage drops when connected with load
Your panel is fine.
A solar panel is roughly a current source over most of its characteristic, not a voltage source.
So, the voltage you see across it depends on the impedance of the load that is connected (or the voltage of the battery that is connected); it isn't set by the solar panel.
The impedance of the load you have is pulling the solar panel's voltage down to 8V, but the solar panel still delivers about 5A under full sun, or about 40W, which is all it needs to deliver for your lamp; it just isn't sitting at its Maximum Power Point.
Here's a (measured) example of a 3W load (a DC/DC converter with a loaded output) connected to a nominally 12V, 10W solar panel:

(Image source: me)