Hello HaPe-CZ,

Thank you for your answer. As you may read on the OSM wiki https://wiki.openstreetmap.org/wiki/Tagging_for_the_renderer, tagging for a renderer is not a good practise. 
So the question is not if the 2 elements are displayed (through an renderer such Mapnik) or not but if it makes sense to put a shelter as inner way from a platform multipolygon. As I already wrote this makes no sense to my humble opinion, as the shelter is onto the platform (as other 'objects' as bin, bench which are also part of the public transport platform). The multipolygon suggests that the shelter is not a part of the platform.
Moreover the platform way still belongs to several train route relations with the role platform, but as the platform way itself (which is an outer way from your multipolygon) must be left untagged (see https://wiki.openstreetmap.org/wiki/Relation:multipolygon) this cause inconsistencies into these train relations. Putting platform tags back onto the platform way is not the correct solution as long as this way is an outer member of a multipolygon (the outer way must be left untagged). To me the only correct solution is to put it back as it was before und delete the multipolygon.

Best regards,

Patchi.