The DC/DC Converter does have it's own short circuit protection, so it is possible that it's tripping out before a fuse can blow. The fact that the headlight comes on briefly would seem to indicate an internal or external short causing it to trip. It likely means that the fuse supplying the converter is ok, and the MBB signal is also fine, as the headlight wouldn't come on at all if either of these were faulty.
I would remove all the 12V fuses, except the one for the side lights, for example, and see if they will stay on. This will check to see if there is maybe an external short.
If the side light stays on, replace the 12V fuses one by one to see which causes the DC converter to trip out. If the side light doesn't stay on, it looks like the converter is faulty, or has suffered water ingress. The converter on my 2017 SR did a similar thing after it got wet, however, when it dried it out fully (took a while), it started working properly again.
That converter is still in the bike, going strong almost 2 years later...
Ulster Zero