Open cell spray foam off gassing 2 years later. How to fix?

My home was built in 2017 and has Accella open cell spray foam in the unvented attic. In spring & summer 2018 we noticed a mild chemical smell coming from the attic on hot and/or humid days. It would enter the 2nd floor living space through the seams of the pull down attic door. It is a mildish sweet chemical smell. Unpleasant but not massively so. It is stronger when standing in the attic.

In the fall/winter the smell is gone from the living quarters. Even standing in the attic during winter the smell is very faint. Still there, but so faint that it isn’t offensive.

It is now Spring 2019 and the smell is back again. The strength of the smell seems identical to last year. That is surprising since I figured it would have off gassed by now.

I noticed a large chunk of foam had fallen to the floor so i placed it in a large ziplock bag and sealed it. A few hours later i opened the bag and smelled the air inside. I did not notice any odor. I then took a saw and cut the piece in half and placed the cut piece to my nose – no smell. Maybe the odor is coming from only certain sections of foam?

I’m concerned about this off gassing and that it hasn’t gotten better nearly 2 years after this home was built. The only thing i can think of is to ventilate the attic by forcing air up there and exhausting air from a 2nd floor window.

What could be causing this smell and how can i solve this problem?