Thanks for confirming. I did add a photo to the first post in the note, though :)

Also, I forgot to update this thread, but the consensus from the discussion in the OSM Portugal telegram chat was that it should not be considered a roundabout if it doesn't have the corresponding signaling (which it doesn't).

And indeed, looking at the surrounding streets, it's not actually possible to, for example, come from Rua Frei José Vilaça, circle the structure and turn left — Rua Marceliano Araújo is one-way, so that would put the vehicle against the flow of traffic.

So yeah, the structure does exist and is round, but it is not marked as a roundabout, and does not work as a roundabout in practice. It's just a green area in the ground, with a lamp post and some signs, that happens to be round.

I think the best outcome here is to split the connection between Rua Marceliano Araújo and Rua Frei José Vilaça into two branches, and draw the circle between them, but not mark the resulting triangle of ways as a roundabout.