Forest raiser state that it make spriggan summon 3 vines but it summon only 2 for me.
My ability tree at the bug occurrence.
I also tried to remove point on forest raiser and only having vine mastery at 4/4 on the vine side of the tree, it feel like the spriggan rarely summoned a second vine but with 3/4 on the vine mastery having a second vine was more common.