I just looked at the code from the in-game console.
The fix is simple. In the enemies tab, the "ai" needs to be modified to use the "tank_on_grapple_latex_plus" move
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on_grapple,tank_on_grapple,tank_on_grapple_latex_plus
priority,tank_on_grapple_latex_plus
depriority,tank_backup
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It was missing the "_plus" at the end of the "on_grapple" line