Geometry checker not removing overlap between features

I have a polygon shapefile layer (CRS EPSG:27700 British National Grid) that has a deliberate overlap and gap between features.

When using the Geometry Checker Plugin (V0.1) in QGIS 3.10.9 I am selecting the topology checks ‘check for overlaps’ and ‘check for gaps’ both with 100 map units sqr.

The geometry checker finds both errors.

I select to fix both errors with the default resolution and the geometry checker goes through the process of fixing the errors, telling me they are fixed and generating a new shapefile.

However, the overlap between two features in the polygon still remains.

Why isn’t this overlap being removed?