Reactive power combiners
there will likely be some routing between the combiner and the amplifiers, so I consider the phase angle of the failure unknown, and possibly a strong function of frequency. The failure could be a short circuit somewhere back inside the amplifier, or a blown wirebond, or almost anything. For sure, adding the isolation resistor will help the remaining amplifiers see a decent match, with the caveat that the resistor does not blow up...
I will take a look at the reflection coefficients that are seen by constituent amplifiers in a combiner, when the amplifiers have random phase and amplitude errors, and see what that tells me..