I would call failing to inform the user a clear defect. It should in the very least supply the information it has. I believe defederating is done within the Lemmy server software, which knows which nodes are defederated. OTOH, it is possible for a Lemmy admin to not use that mechanism and to defederate using a firewall. In that case, the Lemmy software is limited (though it could still say something like “time out trying to reach the other host”).
Admins have reasons for defederating. I am not sure if Lemmy already captures that. There are various levels of effort that could be made in fixing the defect, but I’m calling it a defect because it has info that it is not sharing. I’m sure of that because it’s unlikely that all admins on all the nodes I am on are using just the firewall to defederate. SDF is in fact documented in a 3rd party dataset to be defederating with thelemmy.club.
This is a natural feature of activityPub.
IIRC, other software (e.g. Mastodon) has an API to query for nodes that are defederated and admins have a choice whether to expose the list. Lemmy devs neglected to use that mechanism, but the Lemmy server still maintains an internal list of defederated nodes IIUC.
(update)
Lemmy history and cultural shift
It’s worth noting as well that the Lemmy client has a long history of poor communication with the user. In the early versions, there was a hard-coded slur filter which was a shitty regular expression that often matched normal non-slur words. If you happened to use a word that hit (e.g. you said “grape” and the slur filter triggered on “rape”), Lemmy would simply do nothing when you click submit. No error, and no action. There were many such situations like that and cases where incorrect errors were given. They would often throw away all that the user wrote to give an error, and the back button would yield an empty form. Thus data loss. Not informing users is the style and pattern of the lemmy devs.
A lot of it got sorted out but there is a philosophical deficiet that needs a spotlight. Today’s users are being conditioned to accept poor treatment. There is a broad cultural problem when Firefox users are okay with Mozilla taking the stance that their web browser does not serve the user of the browser but rather the owner of the website being visited. It needs to be turned around so that once again people rally behind the idea that there is only one master and the tool should exclusively serve the user who executes it.