question

@Mig_Muc, where do you have the power supply cable (the little red one that plugs into the shunt PCB) connected, and what fuse is in it? My suspicion is that when your battery voltage drops under 12.0v, perhaps the voltage available to the PCB is dropping under its minimum usable voltage (around ~9v), which could be reasonably expected to happen if the power wire isn't connected straight to the battery, or (possibly, though slightly less likely) if the fuse isn't great anymore (or if it's the wrong fuse). The fuse should be a 1A fuse.

I suggest taking a voltage measurement at the power supply wire and comparing it to a voltage measurement at your battery terminals; if there's any significant difference, then the problem is a voltage drop there -which can occur depending on how/where the power supply wire is connected, and can also occur when the incorrect fuse is in the inline holder and/or if that fuse isn't making a great connection.

If there isn't a significant difference, then it's possible that one or more of the BMV components are defective and should be replaced, in which case you should contact the dealer from whom you originally purchased the device and they will assist you with additional troubleshooting steps and/or start an RMA process for you if it's determined to be necessary.