Hello there! The present is a design choice and not a bug. As the regiment's drill is lowered when the regiment is being reinforced this behaviour is expected. The average army drill for an army that was consolidated vs the army that was not will be roughly the same after they are fully reinforced (not accounting for drill decay, as it will take longer to fully reinforce the consolidated army). If you have any questions about it, don't hesitate to ask about it on the main forum for the game.