Reported this issue before and already moved to duplicate bug, and now report again just want to tell the real root of this issue.

Tested several days, several ways, several skill combinations, and the result is only one.

ISP:
-
Type of Internet Connection:
-
Date & Time:
-
Frequency:
Often

Character Name:
Seille Lorraine

Race:
Hyur Midlander Female

World:
Tonberry, and anywhere inside dungeon

Class/Level:
Arcanist (Summoner) 50

Area and Coordinates:
-
Party or Solo:
-
NPC Name:
-
Monster Name:
-
In-Game Time:
anytime

Steps:
1. Engage battle
2. Use Skill RUIN II while moving
3. Hit Skill Aetherflow as quick as possible
4. Aetherflow gives no stack and no MP recovered, skill cooling down.