If the vertices immediately before and after the ones that are within the polygon are coincident, the solution could be running the Processing > Vector geometry > Fix geometries tool from the processing toolbox.

If the vertices immediately before and after the ones that are within the polygon are coincident, the solution could be running the Processing > Vector geometry > Fix geometries tool from the Processing Toolbox (Ctrl+Alt+T).

If the vertices immediately before and after the one that is within the polygon are coincident, the solution could be running the Processing > Vector geometry > Fix geometries tool from the processing toolbox.

If the vertices immediately before and after the ones that are within the polygon are coincident, the solution could be running the Processing > Vector geometry > Fix geometries tool from the processing toolbox.