A couple patches ago the abilities in the bow skilline were rearanged and as a result the moved skills took the exprience gained of what they replaced. In this case poison arrow switched from the 1st to 5th skill so you will need to level it again to morph it. Annoying yes, but the morph will not need to be leveled to rank 4 again so there's that
Even if you had it morphed it still resets once you respec due to the shifting of skills that occured with the Homestead patch.You just have to re-level the skill again.
Here's a thread of others discussing the issue when Homestead dropped https://forums.elderscrollsonline.com/en/discussion/320100/skill-point-respec-reset-my-poison-arrow-morphs-and-level