Polygon topology validation in ArcGIS - whole study area artefact problem

I can see you still have an error at the lower left side of your image, maybe this is the reason why the whole study area is highlighted as red:

The outer box will always show as a gap. Once all other errors have been fixed, reassess the topology and verify this is the only remaining error in the error inspector. If the outer box is the only area, then dismiss it and continue with your process.