A PRD is written before anyone has seen a working solution. It reflects what someone thinks the user needs, not what has been validated.
Treating it as a build input skips the stage where you find out whether you're right. That's the core problem with a process that goes straight from "feedback" to "PRD" to "build."
The fix isn't more process. It's one more artifact: a working prototype, built fast enough that it doesn't slow anything down, real enough that feedback on it actually means something.
Once that artifact exists, the PRD becomes secondary. The validated product, shaped by real feedback from real people, carries more weight than any spec written in advance of it.