BSP 3.0.0 was released yesterday, but this issue was not resolved.
This is a real shame, as it is an essential feature for running the MG24 on battery power. The following patch is required to use the battery up to around 3.3V.
// \Arduino15\packages\SiliconLabs\hardware\silabs\3.0.0\cores\gecko\adc.cpp
// Add LINE 93
all_configs.configs[0].analogGain = iadcCfgAnalogGain0P5x;