fish, version 3.3.0-2-g210dda2c4

This is a regression from 3.2.2. Tested on Arch Linux and macOS.

function fish_prompt
    echo $PWD $foo
end

function on_foo --on-variable foo
    commandline -f repaint
end

The prompt will not update automatically, but pressing any key will update it.

The issue must be with repainting and not --on-variable, because this works:

function on_foo --on-variable foo
    echo $foo
end