Required for Saving VS Required for Approval

Summary

Change setting from "Required for Saving" to "Required for Approval" but it doesn't work.

Content

We have a User-defined Field that was set up as "Required for Saving". Recently we made the change to have it "Required for Approval". As a result, in the Diagnostic panel, it moved from the "Fields to Create" to "Fields to Request Approval", which is correct. However, when we create a requisition and save it with no value for this field, we can't. The error message says that this field mus be filled before the requisition can be saved. See below picture.

I'm wondering if anyone encountered the same issue before or help me figure out the reason.