I think it largely depends on the culture of the org (maybe share stage, eng vs sales led, what kind of product). It sounds to me like you don't have buy-in from engineering or they're not engaged with your process. What if you asked them to write up a technical spec as a first pass? Then you're both including them in the process and putting the ownership on them to figure it out. I usually view designs / specs as a first prototype, and then you iterate from there.