With regards to allied forces, it's maybe a bug, depending on what was actually intended.
For stacks in the field (outside of the city), if you select one of your stacks, the allied stack will not appear above the Stack Panel as a stack tab, but if you move your own stacks out of the way on the map until you can see the allied stack, you can select it to see what is in it, the same as a stack you own.
For stacks inside a city, it's different, because the mechanism is different. If you have a stack inside the city, the correct number of stacks will be listed (your own, plus the allied), and you can click on the the square next to the city to put your stack into the Stack Panel, but the allied stack will not appear in the stack tabs, the same as with stacks in the field.
Whether this behavior is by design or coincidence, I don't know. Probably the latter.