......... so, gravity hot water then, Rolphy. I guessed it might be. Sometimes, the pressure of the hot (gravity) isn't enough to keep the valve open.
Some mixers manage, some don't. Unfortunately, it's trial and error........ and of course, heads of gravity (height of rooftank above the bath) vary a lot.
Not much help, but some pumped pressure on the hot would probably do it, but it's a bit sledgehammer/walnut etc.
That is, assuming the mixer isn't faulty.