It's not that the inactive trick doesn't work, it is that it is not necessary, and less efficient than doing it right. Let me clarify the one thing Damiac seemed unclear on: the squad MUST have a separate barracks they can train at. Without a barracks to train at, they will do no training at all when active. When inactive, they will only train if they have nothing else to do, and even then, it's hit or miss.
So, the checklist is:
Marksdwarf(s) in a squad with crossbows,
With assigned quivers and ammo,
Assigned to a regular barracks,
Archery targets set up and assigned to the squad, with direction set correctly,
Lots of accessible, unforbidden spare ammo
The barracks they train at does not need to overlap the target. A target is always singular, you can not designate multiple targets for practice from a single overlapping target. Each must be set up and assigned individually, and you should use at least 1 target for every two marksdwarves, preferably one per dwarf.
I have never had a problem with marksdwarves training when I follow the checklist, not one single problem in years of playing. Nor have I ever seen a verified problem on the forums that was not caused by not following the checklist correctly. If it does not work, it is not a "bug," you are doing something wrong.