Entering the following sequence into fish reliably causes the shell to crash on my machine:

sleep 1 | begin ; sleep 1&; disown $last_pid; end

The fish version is 3.0.1 on NixOS (linux kernel 4.20.13, x86_64).
I tested with alacritty and xfce4-terminal, the result is independent of terminal used.

sh -c 'env HOME=$(mktemp -d) fish' does not change anything.

last_pid was not present in fish-v2, so I think the bug was not present there yet.