Strangely, I noticed exactly the same thing just yesterday with one of my 150/35 MPPT devices that had contributed no charge for 4 days, and was showing panel voltage below battery voltage, despite some reasonable (low January) sun on it.
Disconnecting the PV from the controller and I could measure the expected 68v across the circuit (2 x 280w panels in series), reconnect and dropped back to below 24v.
Turned out to be one of the MC4 connectors connecting poorly. Wiggle it about and all would be fine - for a while!
Replaced the connector and crimps, stripping back the cable to new shiny stuff first, and all is good now.