Culprit: https://github.com/jenkinsci/script-security-plugin/blob/fcb41b7e540a6e0a53f89098e1fb6cc5210195a3/src/main/java/org/jenkinsci/plugins/scriptsecurity/scripts/ScriptApproval.java#L637

Source: https://github.com/jenkinsci/script-security-plugin/pull/300#issuecomment-678970315

From Daniel:

As 'user', create a Pipeline, write a script that hasn't been used before, disable sandbox, save.
As 'user', build the job, it fails
As 'admin', approve the script.
As 'user' build the job, it works
As 'admin', delete from approvals
As 'user', build the job, it fails
Expected: Script is now pending approval
Actual: It's neither approved nor pending

It's currently blocking the UI enhancement PR.

Current workaround, you need to re-configure the code that is using the script. Just executing it will not trigger the approval request.