polygonize - QGIS Raster to Polygons produce invalid geometries

I trying to convert a raster layer into polygons with QGIS. At first, it seems there is no problem with doing that, but when I'm checking the output for invalid geometries, it always gives me some invalid geometries (Ring Self-Intersections as shown in the picture). Does anybody know, how to avoid this problem or a workaround for it?

Answer

I found a possible workaround (inspired by Pavel's reply) by just applying the smooth geometry algorithm from QGIS. So I don't have "square polygons" anymore and this means no ring self intersections.

Thanks