MacOS bluetooth audio drops out whenever the audio stream goes silent
I'm confident at this point that it is the speaker's fault. Buggy design. 

Why it worked with one audio source device and not the Macbook I don't know, but it's been reproduced with another audio source since then.

I can't explain how I reproduced it with two different Bluetooth speaker models though. That has not happened since. The two speaker models are by the same brand but can't even play nice with eachother and coexist (selecting one of them connects to the other) so I'm just going to write the whole problem off as cheap hardware/firmware design.