I have setup partial for the “signup-password” prompt (form-content-start) via Management API: PUT /api/v2/prompts/signup-password/partials, with 200 response. But the prompt is not rendering that partial in the signup process.
Allow me some time to search for this behaviour, but in the mean time, just to get this possible cause out of the way, are you using the " Try " feature from the Database to check, or are you performing a complete login flow in an application? This was the issue in this following post on a very similar issue : Custom field does not display on Universal Login Signup page - #4 by jan.clayton
By any chance, are you using the Classic Universal login? The signup-password form will only work with the New Universal Login, as Classic will ignore the prompt and not render it.
In addition, please double-check in case the partial exceeds the 10,000 character limit, as it could also be a reason for rendition failure. Can you try setting something very basic in the partial just to see if that gets rendered, which could indicate an issue with your full rendition that can then be figured out.