SmartSolar MPPT 100/30 voltage high when Lynx BMS is off

I have ensured that it has got to the notice of Victron so I expect that it will be sorted at some point.

Even if remote, you should be able to use VictronConnect via VRM to turn the MPPT off. If the ATC port is free on the BMS you could always connect the MPPT remote terminal to that as another workaround.