This hotkey does not work when playing Abbasid and trying to build siege units on the field with infantry.
The hotkey button is even missing under the icon.
It works when accessing the secondary panel with the Abbasid landmark for instance to get techs and works for every other civ such as Mongol double unit production.
Also, FOR EVERY CIV, this button does not work when selecting multiple types of infantry to build siege in the field, even though BOTH/All sets of the units are capable of building the siege (archers + spearmen). For this button to work, you need to have 1 unit type selected (spearmen) or just 1 unit selected. This might an oversight rather than a bug but I think it’s annoying.
Re-rested on new patch 5.0.9369.0