Thank you for reaching out about this issue and my apologies for the late reply.
After looking into your tenant, I noticed that you selected the New Universal Login Experience but also toggled on the Customize Login Page setting. When this setting is enabled, it will override the New Universal Login Experience with the Classic Universal Login Experience.
If you prefer to directly send your users to the signup page using the screen_hint=signup query parameter, you must make sure that you are using the New Univeral Login experience, which includes turning off the Customize Login page toggle.
I hope this helps!
Please let me know if I can help with anything else.