After investigating the problem more closely, I found that on Catalina and Max 8.1.8 [midiout] and [noteout] (maybe other objects too, but I haven't checked yet) do not react to attempts to set up the macOS build-in DLS Synth as output device when i try to do this by the "usual" method: filling an [umenu] with data from [midiinfo], connecting middle [umenu] outlet to the [midiout] and selecting a device from available [umenu] items (however all other midi outputs except the mentioned one can be selected without any issues).

But, which is puzzling, macOS build-in DLS Synth can be selected from the context menu after shift-click on [midiout].

I also found a nice workaround: if I put the "name" attribute box between [umenu] and [midiout] it also works perfectly.

I answer to myself, but maybe someone will need it too.

Yaniki, I have the same issue; I can't select the AU DLS Synth using a umenu, but I can get it if I double click the midiout object. This is really annoying for me because I have 25 students handing in patchers that need to have the midiout selectable. Thanks for the workaround and the reassurance that I'm not the only one!

Haha... definitely you are not alone. Btw. as I see: the problem is still present after update to Big Sur.