Unstable Phase of Channel Impulse Response
I have two such transceivers ...

With two independent transcievers, there's no reason they should drift together in phase.
You would have to supply both from the same 10 MHz reference signal if you wanted them to stay coherent. Even then, reprogramming one to a different frequency and back again would randomise the phase.
Generally the absolute phase of a channel is unimportant, which is why there's little effort made to control it.