Arcam confirmed this is an issue. The workaround is to set the following setting to off:
System Settings > Timeout = off
If setting that to Off cured the issues, then it sounds to me like the signal sensing behaviour isn’t working properly. That could also explain why the behaviour happened with radio stream to. I’ll add it as a priority issue.
Latest response related to this issue and previous mentioned build:
Of interest to you, however, this fixes the input sensing for the auto-standby feature (the bit was inverted, so it would think there was no signal when there definitely was), which I’ve confirmed myself.