The loss of double click to select all like units in a garrison is a problem that I have encountered since Dom2 for sure and if I remember right, in DomPPP as well. It only occurs when I have really large garrisons, and is irritating, but nothing that I can't live with.
(This morning, I managed to shuttle enough Maenads out of the main garrison box to get my White Centaurs to show up.
)