Welcome, Cerdic.
I have noticed this, but I don't think its a bug. You probably don't normally have more than 50 of one unit type anyways (except maybe longbow or strelet), and if you do, you will benefit from having them in multiple groups. Don't put all your units in one group as a general rule; keep ranged inf in one group, heavy inf in one, cav in one, etc. This way you can assign the units to do specifically what they were meant to do; ie heavy inf engage cav, cav flank ranged inf/canon, etc.
[This message has been edited by christhemean (edited 05-10-2009 @ 02:46 PM).]