Actually I just realized this not too long ago. I restarted the app a few times because of a certain bug, which of course I already reported the said bug, and saw that every items I used before I restarted the app and resumed the battle were lost, or counted as "consumed" to be more precise, even though the battle wasn't finished yet. Pair this with the bug, and it costs me around 25-35 healing pots and attack roots. To clarify things, the bug I reported annoys me like this : whenever I guarded (pressing End Turn) at the 1st fight during a boss stage (no matter how many heroes I guarded), the app froze and forced me to force-close the app.
I think all of the battle items should be reduced and counted as "consumed" after a quest is finished, and not be reduced and counted as "consumed" after every usage during the fight. Resuming a battle means restarting the whole quest, and if item usage really is a must, we will be using the items again and again which results in a heavy loss of battle items only to clear 1 quest.
Restarting the game during a battle means restarting the whole quest, so I think it would make more sense if the item usage also resets back too, right?