RealOEM.com * BMW E53 X5 4.4i A5S440Z MOUNTING PARTS CONTROL UNIT

this link shows pressure regulators which could be the cause but many other things could be the cause
internal wiring
internal components
external wiring to tranny , or anything else could cause issue .
I would need a lot of time to study this to come up with an answer but I can`t test your individual cars to try to pinpoint a solution .
@ Jordo , my X and your X use the ZF A5S 440Z - 4 wheel transmission , they are all similar , the one in the link at Dortmund is similar , all they did was a proper transmission service replacing a few parts with updated parts . kits are available .
read the pdf link in my previous post , there is good insight into how the torque converters work and all the internals.
I think you will come up with something good out of all this .