Please tell us which fish version you are using by executing the following:
fish, version 3.1.2

Please tell us which operating system and terminal you are using. The output of uname -a and echo $TERM may be helpful in this regard although other commands might be relevant in your specific situation.
Linux t430il 5.8.0-2-amd64 #1 SMP Debian 5.8.10-1 (2020-09-19) x86_64 GNU/Linux
st-256color

The issue does not occur in the no-customizations setup, where I do not get any TAB completions at all. I have an

alias ssh="env TERM=screen command ssh"

but removing it (functions -e ssh) does not change the situation. I also tried to mv my ssh config out of the way, also without any effect. That's when I was a bit stumped --- what could go wrong here?