It appears that the system treats signer email addresses as case-sensitive during the signing process. In our tests:

When the email address is sent in lowercase, the signing page correctly recognizes the predefined signature field and shows the “Start” option.

When the same email address is sent with uppercase characters, the signing page instead prompts the user with “Add Signature,” indicating that the predefined signature field is not being recognized for that signer.

Based on this behavior, we assume that email casing may be the root cause of the inconsistent signature interaction.

From our side, we will standardize the signer email addresses by sending them in lowercase format in the API request to avoid this issue.

However, we wanted to share this as an additional finding, as it may be beneficial for the platform to normalize email addresses internally to avoid similar issues in integrations.