This post has 84 likes. When I clicked it, my like went away, but the number stayed at 84. I was then able to re-like it to get it to 85. When I refreshed my page, it went back to 84.
'tis as old as the forum
It’s a discourse thing, and I don’t think they’re going to fix it. It’s just a harmless visual bug - the server isn’t registering the like (which is why it goes away if you reload the page).