This is working as designed.
Manpower in Imperator doesn't represent every man you can force into the army* like in later era Paradox games - it is every man in your realm who is willing to join your army for money. If you disband a cohort, the people find something else to do than wait around for your call.
Don't disband your troops if you need them.
* If you want to levy troops, you need to play the Celtic barbarian traditions or the Achaemenid Persian ones. That'll allow you to raise LI and LC for 50 military power and 0 manpower, as many units as you've got cities in the province you raise levies in (on a provincial cooldown.)