All right… so it works. Thanks for your test.
I made something, what’s not really “nice”. I put a “simple” way with the tag “natural=water” over the MPs… it isn’t really a nice solution, but like this, all programms can show the lakes.
It’s not nice indeed. What should a doubly drawn lake be good for? If you can draw it with a simple line, you don’t need the multipolygon and vice versa!
Obviously someone thought that a multipolygon is needed, so you should check first what it is used for. If the multipolygon has to be used to represent something in OSM and ArcGIS can not handle multipolygons - screw ArcGIS (or change the shapefile export algorithm). Becaue if it’s allowed in OSM, it will be used. Next time it’s maybe a forest with a clearing in the middle or something else that’s not so obvious like a big lake…
In this case, the multipolygon is also used to draw a breakwater near Faoug - which is gone with your simple way.
Luckily the Murtensee is just divided in to a multipolygon because of the boundary-ways - which should be tagged as a separate way anyway. So I corrected both, the boundaries and the lake. It should be correct now in OSM and in ArcGIS.