Arsene has a special "treasure preservation box," which is one of his phantom thief tools. This box is infused with intense preservation arts; Arsene needs to place the items inside immediately after stealing them.
Otherwise, during any form of time-space crossing or high-frequency movement, or if struck by spells or curses from others, the thin layer of preservation arts around it might be interfered with and shattered by his transcendent powers, potentially damaging the hard-earned items he stole.
Ultimately... it's because "preservation arts" itself is regarded as a form of extraordinary power. Therefore, it also has a priority.
The preservation arts can act as a shield to defend, as it can counteract spells of lower energy levels than itself; just like it can be shattered by spells of higher energy levels, the preservation state applied to anything will instantly break upon contact with any transcendent power higher than the spellcaster's energy level.
