First, there are many different types of coax cables that people call LMR400. Some have a shield that consists of aluminum foil over copper braid. Some, like the M&P 10mm stuff have copper for both layers. LMR400 is the Kleenex of coax.
Does it make a difference during full duplex transmissions?
Darned if I know, but I’m certain that this range problem isn’t because of LMR400.
I just read an older thread on another forum where a guy said “my company has several repeaters that work just fine with LMR400, but I’m looking at making a portable repeater and I want something more flexible.”
Immediately someone went into great lengths telling him not to use LMR400 for repeaters.