The input of the amplifiers are active components and need to be switched on to do their job. Obviously when one amp is switched off it is providing low resistance to ground (or frequency dependent resistance) and affecting the line level signal adversely.
The fact is that this is rather a dogs breakfast approach to hooking up audio gear. It may kind of work with some gear depending on the different ways the input circuitry behaves when switched off (due to different designs)
Honestly you need a Zone Mixer between the output and the two separate zone amplifiers as it appears currently you are just connecting the same output to two amps (dogs breakfast approach)
The dogs breakfast approach could also lead to ground loop issues and other interference/noise but on the positive side perhaps different interconnects may sound different in such a setup - providing endless entertainment going down that rabbit hole. (as I have stated many times - well designed gear that is correctly setup won’t need special cabling)