Your panel is very probably fine (no guarantees, though).
A solar panel is roughly a current source over most of its V/I 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 itself.
The impedance of the load you have is pulling the solar panel's voltage down to 8 V, but the solar panel still delivers about 5 A under full sun, or about 40 W, which is all the power it needs to deliver for your lamp; it just isn't sitting at its Maximum Power Point, where it could and would deliver more (and, in this case, too much) power.
Here's a (measured) example of a 3 W load (a DC/DC converter with a loaded output) connected to a nominally 12 V, 10 W solar panel:
(Image source: me)