The Web Mercator extent being used for the tiles is incorrect.
In MBTilesLayer.cs:
Replace line:
var envWeb = (Envelope)GeometryEngine.Project(envWGS, SpatialReference.Create(3857));
With:
var envWeb = new Envelope(-20037508.3427892, -20037508.3427892, 20037508.3427892, 20037508.3427892, SpatialReferences.WebMercator);