Withholding tax sometimes not by default show on invoice

Summary:

Withholding tax group was setup under the vendor site assignment and expect to always default to invoice. Normally vendor will directly send the invoice through isp, but we observed the inconsistent behavior that while the WHT field on invoice will auto populate, sometimes it' left blank.

Content (please ensure you mask any confidential information):

We have the withholding tax detail setup on vendor site assignment, and the 'use withholding tax' option is checked on the vendor profile - income tax. And since in many cases the WHT record can be defaulted on the invoice, we would assume this setup should work. However, sometimes when vendor sent invoice from isp, this record wouldn't auto populate, and user need to manually check each individual invoice to make sure the tax is available, which create many effort. Wonder other than below setup, what might be some reason that can cause this inconsistent behavior? Thanks!