Maybe I'm not understanding bioassemblers, maybe they're actually broken, all I know is I was busy trying to fend off some tanks on Castra in remote view then came back to find the bioassembler had died and become a ghost. Isn't there supposed to be some kind of revival system for them?

Happy to provide more details if needed, but the first question is simply "am I missing something here" because as it stands it looks like I'd need to have automated production of bioassemblers just to replace exploding bioassemblers.

Bioassemblers by default lose 3 health per second while idle and 1 health per second while producing, at which point they become 'dead' bioassemblers that can be revived with 20 nutrients. Dead bioassemblers lose 4 health per second, so you need to revive them within 25 seconds or they'll permanently die.
Maybe your nutrients throughput was insufficient so it died before it got enough? Tough to say without knowing the layout of your base

Ah, that's entirely possible since I was distracted by Castra in that moment. And it was my first bioassembler that's supposed to be starting my Tellus base, so almost certainly my nutrients were lacking. I rolled back to an autosave either way, and I'll make sure to deal with Castra issues before planting stuff on Tellus!