[werk_ag]

I have mounted the panel flat like mentioned in the forum to do, but it is under a cover.

As AllyCat said, at first sign it seems an electrical failure, and it can be the PV cell itself, but it also can be due to other reason. You said that you have the PV cell inside a closure, which is good to protect the PV cell, but whatever the closure you are using, put the PV cell directly under the transparent side, with no space between them, not just near, or at the middle.
Don't use teflon or any diffuser to cover the PV cell, the calculations for the shunt resistor are made based on a fully exposed cell, however it can be good for the UV sensor (I haven't tested it covered ).
Could you please post a picture of the top side of the solar sensor?

As a thought to solve this problem, I could try mounting the panel out of any protective clear housing, so that it is fully exposed to the sun. But how will the panel handle this over the long term?
Is it going to survive being rained on and in full sun light?

From my experience, the epoxy that covers the light sensitive material is not very resistant to weather conditions, and after a year it starts to lose the glossy aspect and turns opaque.
The best material I found to cover the PV cell is a thin glass. After more than two years of continuous work , the surface of my PV cell still is glossy as new, and working perfectly.