Test on main branch gives a bit different result, but still buggy. Everything seems to work correctly from the attacker's point of view, but the dying defender does not get removed from the defender's client (I assume this one, at least, to be a server side bug - server not sending the unit removal info)
Some more trouble with hiding extras -> #47655
When I try to use an extra with the option unit_seen = "Hidden", the client (I tested gtk3.22) does not seem to handle it: when an enemy unit tries to move/attack to the tile, it is not possible and it appears the message:
"Orders aborted as there are units in the way"
Sandbox ruleset (I tested 3.0.6) includes a fortress with this "hidden" option enabled that allows to reproduce the bug:
1) place some units inside a fortress (being the unit and the fortress from the same owner)
2) try to attack them with an enemy unit. In my tests it is not possible. I suppose the expected behavior was to be able to attack them, but not knowing the remaining units, as when attacking a city.