The Era 300 isn’t really designed to be grouped with the Beam while playing TV audio. And your network can’t seem to handle sending the TV audio to the Era 300 without dropouts. Grouping speakers to the Beam while playing TV audio will also likely result in a delay from the grouped speakers which can cause an echo effect.

It also (in addition to @GuitarSuperstar ‘s answer) suggests a certain level of wifi interference in your network, since the line in on the TV requires a little bit more bandwidth than regular music streams tend to. Probably just enough to interfere when playing a grouped TV experience, but not enough to nudge a music stream.