Hmm, I was thinking about an ecm fault because the diode is only needed for those brake controllers that turn on the brake lights when the emergency brake function is used or as a preventative if a controller fails and appplies power to the trigger line, as either would produce similar results to a faulty brake light switch. Because of your ecm's previous strange behaviour I'm even more favouring an ecm fault although it is possible that LR have introduced logic and a fault code for illogical brake light switch operation.

Fitting a diode would be prudent although I haven't fitted one to mine yet. As mine has just had the latest ecm s/w applied, I will fit my P3 and try CC tomorrow or perhaps later today although without the van, the P3 wont try to operate so most likely of no help.

Edit: Tried the P3 + CC but without the van, even using the brake lever without any abnormal behaviour. It needs the van attached to get a comparative result though.