Since this is not the first time this happens, I’m reporting here.
When someone (known) likes a post, sometimes it appears as “unknown/deleted user”, although, in this case, this is a personal message and the person is actually known.
It may be a regression in a recent release.
If there is a way to rebake post or something like this from the console to fix it, I’m all ears!
Have seen that happen with a user that has been muted/ignored, which seems to probably be intentional, but that is happening with a user you have not muted?
Thanks for asking @Architect.
I have seen this happening once or twice previously, but in this case this is a private message with a couple other people, and the person who liked the post was sitting next to me, so I could confirm who performed the like. We’re both admins on the forum, so there’s no chance we muted each other. 
This has been a persistent issue for me as well, whether the post is public or a message. Like @hellekin I also have higher privileges on my forum (but mod, not admin).
The only liked-by users who do show up are other moderators/admin. But I’ve noticed it’s usually just one, so if 3 mods liked it, sometimes only one icon will appear and all the others say “unknown/deleted user” when you hover over the icon. Not consistent though…
The issue does not occur when logged out.
