There is an inconsistency how the two parts of the Rambla de Mar pier are handled. Comparing the two parts in Nominatim - "upper" [1] and "lower" [2], you will see that the "upper" one is part of the Gothic Quarter neighbourhood (and in turn part of Barcelona and Catalonia), while the lower one isn't.
Also looking at the borders of the Gothic Quarter, Ciutat Vella and even Barcelona - none of them actually contain the pier (not even the "upper" part).

[1] https://nominatim.openstreetmap.org/ui/details.html?osmtype=W&osmid=231284638&class=man_made
[2] https://nominatim.openstreetmap.org/ui/details.html?osmtype=W&osmid=231284639&class=man_made