I think this is an effect of just the reeds being so massive and big so that they can make the deep sound they make.
Think about pressure vs. pressing a note. On a normal pitch melodica, its not unusual to get a degree of expression to your notes by bringing the air pressure up when you wish to play a note. It is very natural to use the breath this way. This makes the attack of the note sound just a bit softer. There isn’t a particularly noticeable delay, but if you compare your technique when playing staccato, you’ll find you’re getting a sharper start on the sound by bring the pressure up first then releasing it with a sharp keypress.
I too have noticed the lack of attack when playing the Melodion B24 – but find I can address it with steady pressure and sharp engagement of the key. The sudden air pressure burst when the key starts the reed vibrating quickly. Its literally a variation in your breath control against keying patterns.
I think you’ll find that getting sharp attack on the low notes of an accordion is easier with faster actuation of the valves and firm pressure against the bellows – big reeds simply require more and sharper air pressure to sound quickly.