I have similar problem, I also mentioned it in here, Source of the post
The tiles cover the AOI have been downloaded from here source :https://tiedostopalvelu.maanmittauslaitos.fi/tp/kartta?lang=en
Both are opened in QGIS and then exported as geotiff WGS84 (geographic coordinates) and mosaic both tiles, Would you please to bring my attention to any not correct step!
The problem is, the slave is empty aftermath corr.