UpdateMapper.getMappedObjectForField(…) invokes the very same method of the super class but handed in an already mapped value so that value conversion was invoked twice.
This was especially problematic in cases a dedicated converter had been registered for an object that is already a Mongo-storable one (e.g. an enum-to-string converter and back) without indicating which of the tow converter is the reading or the writing one. This basically caused the source value converted back and forth during the update mapping creating the impression the value wasn't converted at all.
This is now fixed by removing the superfluous mapping.
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