@givemersspls I've read up some source material and have come to a conclusion that it was a Stats thing. While it does cost mana to maintain Dragonification it seems that she can somehow recover even while in dragon form on the basis that she's inactive/asleep, compared to Dragon form while in combat.
as "Human" while in "Dragon" form
Strength 770 +4620
Vitality 1100 +6600
Defense 1100 +6600
Agility 580 +3480
Magic 4590
Magic Defense 4220
Her superior stats alone should justify why staying and sleeping in Dragon form would be optimal or safer even, It's also noted that normal Dragonmen have immense will and the fact that a Dragonman of her caliber was overpowered so easily was even surprising by Isekai World Logic. The Caster was described as very proficient or even some sort of Genius with Dark Magic also it did not help that the caster had nearly a whole day to do it because she was sleeping. (It's a apparently a racial trait that Dragonmen are heavy sleepers.)
It's also mentioned that due to her mission needing to be swift and because traveling long distances/flying over oceans was an exhausting ordeal, she made the blunder of placing herself in a deeper sleep compared to normal while stopping for a rest.
Source: Arifureta volume 03 – Chapter II: A New Meeting