If you create a custom advancement that is supposed to trigger using the location trigger when you find a structure that uses internal structure-block structures to generate (i.e. any structure added in 1.9 or later, or an End City, Igloo, Woodland Mansion, Shipwreck, Buried Treasure or an Ocean Ruin), in a similar manner to vanilla advancements "A Terrible Fortress" or "Eye Spy", well, it is inconsistent as to whether or not it will actually give you the advancement when you enter such structure.
While the advancement will show up in the advancement menu, when you enter the structure in question it might not actually work.

Meanwhile structures added prior to 1.9 (like Nether Fortresses, Strongholds, Pyramids and Jungle Temples) always work seamlessly.

1.9 was the version when they started making new structures added to the game use structure-block structures to generate, and due to this strange pattern it makes me think the issue has something to do with this.

This means "The City at the End of the Game" advancement is bugged, and any new advancement created for these new structures also works inconsistently.

Code for the criteria component:

    "criteria": {
        "ocean_ruin": {
            "trigger": "minecraft:location",
            "conditions": {
                "feature": "ocean_ruin"
            }
        }
    }