..... isolating it from all other circuits?
One circuit being isolated from all other circuits may be explainable by something written years ago by a X10 industry insider (might be a Phil Kingery, Dave Houston, etc...I can't remember).
It goes like this: Oxidation occuring between the disimilar metal of the circuit breaker buss clips and the panel's buss bar can act as a 100kHz attenuator. High current 60Hz gets readily coupled, but high frequency does not. "Stranger than science"... I don't know if it really happens or not. If it really can happen, wiggling the breaker or or disconnecting from the buss and then re-inserting will solve. FWIW back in 1990s in another home, I removed all breakers and sprayed the buss with Cramolin Red..that did indeed improve X10 communications throughout the home. So maybe there is something to it.
However even if that were happening, it would not stop X10 communications within the branch.
It is beyond any more guess work from me, but we all would like to hear if you solve this.