Contact's TAX ID wrongly deleted in admin area

[Remitur]

There's an issue about TAX ID management of contacts in admin area; yet reported to WHMCS's staff, they answered that "it's not a bug, it's  the default behavior of
the system"

But it's still an issue, and so let me show it:

If in /admin/setup/payments/tax the "Tax Support" is ON, everything works fine.

If the "Tax Support" is off (or if you're using your own "tax support", as i.e. the one provided by @Kian 's Billing Extension),  you can experience following issue:

1 - user in client area can create contacts and insert also for each contact a "tax ID"
2 - if admin (in admin area) goes to edit this same client contact, saving changes the "tax ID" field will be simply cancelled (without any warning, nor any way to recover it)
3 - again: if admin in admin area try to edit tax id of a certain contact, this data will not be saved   

Workaround:

If you have TAX SUPPORT off, do not use the tool in admin area to edit customer's contacts.

Instead, login as user and use the customer's interface in client area to edit it.

Annoying, but it seems that "the system is functioning as intended" ... 🙄

Edited December 6, 2019 by Remitur