recently, i realised that the sieve filter i use to mimic the “Set handling for tagged mail” setting “In subject” stopped working. while looking into it, it turned out that the filter was neither throwing errors nor had it stopped all work. indeed, the only extension that didn’t work was the editheader sieve extension.
in the end, it turned out that the ‘duplicate’ extension and the default duplicate detection interact in a (for me surprising) way: a changed mail, for example, because of an addheader, is considered a duplicate of the original unaltered mail and therefore gets discarded. i tried this with something very basic like addheader "X-Foo" "Bar";.
i think addheader/deleteheader in sieve filters could be useful not only for mimicking some functionality… so, maybe this bit of information is useful to someone - or someone can help me out to understand why this works-as-designed (and also how to use the two extensions all together in a filter).