I have a Multiplus 2, its reading battery voltage when charging as 3v higher than at its terminals on the unit. So Multiplus battery terminals read 52.9v and battery reads 52.9v, but the victronconnect app shows 56.35v, and when charging it goes to float fairly quickly as a result. Strangely though the low voltage cutoff works exactly at correct battery voltage so its just the charging side. I tried to update the firmware on this 6 years old unit from v448 to v560 to see if it would fix the irregular voltage reading, but the unit just turns off at start of firmware update.
The VictronConnect app has a number 2629 in the product info section and there is a firmware version 2629560 but wasn’t sure if it was the correct one or not to try and VeFlash.