Didn't even know about process substitution, had been using fifo's to achieve this!
By default, Fish actually runs the processes in a strict sequence. But this is to avoid the above deadlock situation. And it therefore isn't process substitution.
This SO answers a bit about the issues with FIFOs and fish shell. Basically it comes down to evaluating redirections before fork instead of after, which is because fish uses threads. https://stackoverflow.com/questions/61946995/ls-fifo-blocks-...
Does this mean that every time you direct output to a file, like so:
the_prog > the_file