Shapefile in leaflet shifted North 0.25°

I have the following problem:

I render a bunch of weather maps as PNG overlays with grads and then reproject them from EPSG:4326 to EPSG:3857 to display them in leaflet as follows:

gdal_translate -of "GTIFF" -a_srs EPSG:4326 -gcp 0 0 -40.0 80.0 -gcp 4800 0 80.0 80.0 -gcp 2400 1000 20.0 55.0 -gcp 0 2000 -40.0 30.0 -gcp 4800 2000 80.0 30.0 ${INFILE} /tmp/SK_L_referenced.tif
gdalwarp -overwrite -setci -t_srs EPSG:3857 /tmp/SK_L_referenced.tif /tmp/SK_L_warped.tif
gdal_translate -of PNG /tmp/SK_L_warped.tif /tmp/SK_L_warped.png

Those reprojected PNG in leaflet match the maps that grads produces itself, i.e. they should be correct.
Additionally, I reproject some shapefiles which I also produce with grads as follows:

ogr2ogr -f "ESRI Shapefile" output.shp input.shp -s_srs EPSG:4326 -t_srs EPSG:3857 -overwrite'

Those files appear in leaftlet shifted ~0.25° to the North. I have no clue whatsoever what could be the problem here. It’s a total mystery to me.