My Playbase is doing the same. Not sure if it was from the latest Sonos update or an Amazon change. 

Can you check the Alexa Group setup for the two devices.  I’ve noticed in my set-up my soundbars are still listed within the preferred speaks but have dropped from the group and had to be re-added. (To enable ducking, the Beam needs to be included within both sections.)

I’ve experienced the same issue just recently. My Beam now ducks when I wake my Dot in the same room. It never used to, and I much preferred when it didn’t duck (it’s one of the reasons I put a Dot in the room in the first place!). Does anyone know how to turn off the ducking? If I’m quickly telling Alexa to dim the lights during a movie, I don’t really want the sound to duck out!

Ran into this same issue as of lately. I have an Echo Dot in the room that I’m using exclusively for Alexa requests because I don’t like the ducking feature, and the latest update completely undid the solution I'd been using this whole time. If I wanted to drop the volume on my TV when asking Alexa to do something, I would un-mute the microphone on the Beam lmao. 

I would love to have ducking to be a preference that can be disabled. More often than not, Alexa has all ready executed the command and we’re waiting around for the sound to come back up on the TV. I’m sure some folks like it, but for us it’s a completely unnecessary feature.