[Rendar]

If you use arrows (in this instance, gust arrows) and finish a battle with some left over. They carry over.

If you then go to use the same type of arrow next battle to renew them, they'll last a turn (maybe) then cancel out next turn (or when you use them again) and you'll have to spend 3m again to get them back.

[Neus]

Arrows carrying over is intentional.

Using a special arrows skill that already gives the same arrow type you have loaded causes you to 'load' normal arrows.

Neither of these are bugs.