Table trigger - behave differently when insertion from app vs UBE

We are on JDE 9.2 for tools release.

There's a custom trigger on F0411 that validates purchase order (PO) field against F4311 before insertion into F0411.

When we run R0411Z0A to create a voucher with no PO value, the trigger will fail and not allow any insertion into F0411.

When I input a voucher with no PO value using the application (P0411), I am able to create a voucher in F0411 without issue.

Why would trigger on F0411 behave differently?

Thanks.

Here is the trigger coding: