The problems is I guess with comm or power wires or sense wires connected to relocated boards not boards themselves. you were going through rough terrain, bumping ????
Spare hub battery has Nickel cells not your A123 battery, yes your spare BMS works fine with display/controller of course.
or shorts ,not necessary well , depends how nice are those BMS relocated, no shorts, connectors power and comm nicely isolated?
Boards no shaking, well secure, all this incorporation of EPLUS BMS into your battery is very important.
light rider and pumped tires.

EDIT:
As I found out later EPLUS display/controller going blank can be caused by switches on display/controller, tactile switches made cheap, shoddy membrane falling causing shorts. try pressing power button while riding for long enough and what happens???