I just had some trouble converting a set of tiles I scraped into a nice package. I tried both mb-util and tiles2gpkg_parallel.py. Both seemed to work but QGIS did not display anything. The “solution” was to convert the tiles from 512×512 pixels to 256×256. WTF?
tiles2gpkg_parallel.py also crashed at joining its intermediate files if used in Python 3.6. It needs “-tileorigin ul” for OSM-like tile names.