I can confirm I’ve been having this same issue for years. Other folks can link their mounts just fine, but when I attempt it, it just comes out as a string of gibberish. It’s been a problem since at least Shadowlands, and it’s not related to any of my addons.
I am glad I am not the only one, but alas it doesn’t bode well for me reading that you’ve been suffering with this problem for years.
It’s only just happened since the patch 11.0
I’ve done everything you’ve suggested few days back, nothing worked
It is only my main character who is suffering this problem, all my other alts are fine