This boundary is open by the coast side, as many others (¿all?) in Spain. Something similar to the close-gaps in the sea generation could do the trick.
That's not a nice job to do... We might continue the boundaries along coastlines. This would be a job for a specialized BoundaryRelation class that inherits from MultiPolygonRelation.
I have finished my checks with Spain and the result is perplexing: The splitted tiles do not contain any boundary tag. I have used splitter r164. The .pbf file does contain them. So it seems to be a splitter bug (or I have used it wrong).
Ways that cross tile boundaries may be incomplete. When generating a tile, the splitter includes in each tile all of the nodes in the tile, all of the nodes within OVERLAP of the tile, and then any ways that use any of those nodes and finally any relations that use those ways or relations. If a way denoting a boundary has widely spaced nodes, where none occurs in the overlap region, the way will have a gap between the the last node in the region and the border. At present, it doesn't go back and 'fill in' missing nodes in those ways/relations, and it has never had that functionality. Doing that would require additional passes, but I think it could be added in a few days of work. Scott