screentime can be unlocked on ipad with ipad PIN and not screentime pin

Read:

If you forgot your Screen Time passcode - Apple Support

It is designed that if you want to reset the code you need an Apple ID and password.

Make sure you haven't set the phone's passcode and the screen time passcode to be the same. If you truly think it is behaving the way you describe then contact Apple.

Contact Apple. Click here --> Choose your country or region - Official Apple Support Select your country, then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

Contact Apple for support and service --> "See a list of Apple phone numbers around the world."

Click here --> Contact Apple for support and service - Apple Support

Basically what you say you see isn't something I have ever encountered before, nor does it fit in any way with how devices work in my experience. Since you seem to have a unique problem I recommend you contact Apple about this using one of the methods I provided earlier.