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This feels like it is intended to counter body vaults in the sky. Except all you need is an alt or a friend to do the other method.
Bug report sent through the Help Center and updated with testing on single player, the spell performs as expected. Speculation that the issue is related to the lag and server performance.
Can you post a write up of your test? What did you use for measurement? And your results?
I went into single player, levelled sorcery until I unlocked Call of Nergal spell. Went to an open location (in this case in front of the ob at Khael’s stronghold, near Sep). Cast the spell, bat came, I latched on and fly up into the air. Once I was in the air I pressed space bar to ‘fly’, bat flapped it’s wings and flew higher. I repeated pressed the space bar and the bat continued to gain altitude. I did this until the bat was out of stamina.