CCA device lost in iOS app if using CCA group

[Nyarly]

Plan

Premium

Country

Germany 

Device

iPhone 7, iPad Air 2, Chromecast Audio

Operating System

iOS 11.4

My Question or Issue

I'm using two Chromecast Audios. If I cast to each from Spotify on my iPhone or iPad, everything's fine. If I create a CCA group, however, one of the CCAs goes missing from the device selection. It never returns, neither when I reinstall the app, nor if I wait 2 days. It's gone both on the iPhone and the iPad. I can cast to the group from Spotify and both CCAs play, but I can't cast to the individual (missing) device anymore. The Google Play Music app sees the individual devices and the group (see screenshots).

I don't know where to post this, here or in the CCA/player forum?

[Nyarly]

A similar issue with cast groups is also described here.