This is from a thoughtful reply on reddit. The poster is replying to a low-information solar user and so has simplified his answer.
Solar [panel] voltage doesn’t really tell you much
It doesn’t tell OP much, but it can reveal much after we learn what the controller is doing.
A glance at a typical panel power curve might help them see that “high panel voltage” does not mean “high power output”. Quite the opposite – once voltage climbs past Vmp the panel’s power output drops dramatically. MPPT takes advantage of this to tweak power output as needed to hold a given setpoint. Therefore, Vpanel > Vmp suggests the controller is intentionally reducing panel output.
For any whose eyes have not already glazed over, I’ve written a guide to interpreting what solar charge controllers are doing. It leans heavily on Vpanel observations.