I had this 2-3 times now. On my side it says in chat that I sent the request, the other person confirms that the option to block guild invites is not ticked either, but they simply don’t receive a request.
I seem to remember that one person really tried and eventually it worked, but I don’t remember whether we knew why. (I also recommended a /reload but dunno whether this time the player tried it.)
Is this another UI bug or such? Maybe with addons on their side?
Likely to be some random addon issue, addons tend to cause 99% of weird issues.
However, in saying that… recently I have noticed that upon logging in there has been something weird with guilds where they are unavailable for a few minutes and there is a message in the chat window saying something like “services currently unavailable” so perhaps it is linked to that.