https://bugs.winehq.org/show_bug.cgi?id=43725
--- Comment #1 from aaahaaap@gmail.com --- Judging by the info here https://wiki.winehq.org/MIDI#Selecting_the_Output_-_the_MIDI_mapper I think this is just expected behaviour. Especially the sentence "In Wine, the mapper forwards to the first device (number 0) by default." seems to indicate this happens automatically. I've also been able to get a usable port by adding snd-seq-dummy/Midi Through as the first port, this one I can actually use with software synths.
It does seem like the docs are out of date, since there's no screen winecfg that shows the available devices as mentioned in these sentences: "The order is shown in winecfg, with device #0 occuring topmost. If this happens to be the "MIDI through" port on your system with SW-only synthesizers, you need to change the configuration via the registry."
It would be nice if there would a configurable option in wine to choose which device to use.