To reproduce: select any relation object, and click the big button at the top with the type of the relation, e.g. this one:

In the list of available relations, scroll to the multipolygon, and click the (i) next to it:

The "edit/translate" link in the above image actually links to the data item for the key=undefined
Instead, it should perform a lookup for Relation:multipolygon, and get Item:Q16053.
Might be related to https://github.com/openstreetmap/iD/issues/5861
I just fixed this 馃帀

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I just fixed this 馃帀