It's occurring for us approximately 30 seconds after connecting. Occurring across android (galaxy s5) and iOS devices (iPhone 7 plus / iPhone 6 Plus)
Rebooted internet connection, reset CCA to factory, rebooted all networking gear in the house, still occurs until I logged into here and then went to verify how long it takes to cut out, and it's just finished the first track and onto the next.
Since the last couple of weeks I face similar issues when casting Spotify from my Android devices to my Chromecasts (mine are as well under 1.21.74816):
estblishing the connection from Spotify to my Chromecasts takes longer than before
music stops from time to time, randomly, during songs (i.e. not when changing from one song to another)
if changing anything on spotify on my android device (e.g. moving to a different song in my playlist), this will change on the Spotify application but will not change anything to the music played on my Chromecasts
Adjusting sound on my Android device when on the Spotify app will not change anything at all - the volume indicator on the Spotify app would not even show up or change
Music keeps on playing despite having closed the Spotify app
Note that Spotify behaves normally when casting via bluetooth and not via Chromecast. I am not sure if the issue is relate to ChromeCast or to Spotify, but considering the behaviours described above I suspect it is related to Spotify not handling anymore correctly the connection to Chromecasts.
I am having the same problem. I found a workaround. Instead of trying to reconnect as usual, i go into settings of spotify and I go into Connect to a Device menu. When I choose my chromecast audio from there instead of from 'under' the song playing menu, it's working.