The solar controller usually has a display showing output amps and battery voltage. Some also have input info.
Is this one of those smart shunt, bluetooth deals? If from the shunt then that would be amps to the battery after any loads are taken care of, but I don't know how those work exactly. Might help to explain the set-up and a link to the Victron where you are getting the readings so we are talking the same language.
It takes about two minutes or so for the panels to heat right up when first put in the sun. Could your Pmax reading have been when they were first exposed to the sun?
I doubt that 99% efficiency for 24-12. Perhaps for 12-12 or 24-24. Are there efficiency curves for your controller? Ones I have seen show lower efficiency -mid 90s- the bigger the voltage diff between in and out for the buck converter in the controller.