Filter rivers with Osmosis

I'm trying to filter rivers, highways and water from a planet file in Osmosis as follows:

osmosis \
--read-pbf-fast file="planet-latest.osm.pbf" workers=8 \
--buffer \
--tf accept-relations natural=water waterway=riverbank highway=*  \
--used-way \
--used-node \
--write-pbf planet-RELATIONS.osm.pbf

osmosis \
--read-pbf-fast file="planet-latest.osm.pbf" workers=8 \
--buffer \
--tf accept-ways natural=water waterway=riverbank highway=* \
--tf reject-relations \
--used-node \
--write-pbf planet-WAYS.osm.pbf

#MERGE RELATIONS AND WAYS
osmosis \
--read-pbf-fast file="planet-RELATIONS" workers=8 \
--read-pbf-fast file="planet-WAYS" workers=8 \
--merge \
--write-pbf planet-MERGED-highway-water-riverbank.osm.pbf

After that I turn the .pbf into GeoJSON with Ogr2ogr and then into .mbtiles with Tippecanoe. The highways and water work perfectly, but the rivers are messy: parts of every river works well while some parts have broken polygons, and others parts are shown as multilinestrings even though they should(?) be polygons.

If you have any pointers on how to filter rivers properly I'd greatly appreciate it! (If this is the wrong forum for this question please redirect me!)