Hey everyone!
I’ve been struggling with the exact same problem as @shadowhand since my first steps with Discourse a few weeks ago. For me, it is still not resolved on current beta.
After having a look at message_bus.rb, I think the problem is as follows: The default keepalive_interval is set to 60 seconds and the subscriber is killed after 3x this interval, i.e. after ~ 3 minutes. The asset precompilation for some files takes about 5 minutes on my DigitalOcean droplet.
Without having further investigated the asset compilation step, my guess would be that while a single asset is compiling, the subscriber is blocked, can’t respond to the keepalive and… gets killed mercilessly 
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To confirm this, I’ve set the default keepalive_interval to 0, which disables the keepalive mechanism. Asset precompilation then runs through as expected 
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Maybe a solution would be to disable or raise the keepalive_interval for the asset precompilation step? Anyway, hope this will help you to fix this bug.
PS: Thank you for your awesome work on Discourse!