Today the up/up/dn/dn/up/up/up/dn/dn/dn volume sequence that allows simultaneous output to speakers and headphones plugged into remote stopped working after months of working fine. This seems to correspond with an update to 9.4.0-4174-46 today.
Follow-up.. I swapped out another ultra (same model) and it worked - so not inherently due to update. Also observed following, on the final <dn> press, it does not lower the on screen headphone volume (which appears to be the normal behavior when recognizing the code) and there is a slight sound that it "tries" to output audio, but nothing.