No VAT added to Shopify orders on 'random' cases

[svespinup]

We have a Shopify Plus store with a main International Market with default prices in EUR and a Secondary market for UK with prices in GBP.

Our issue: for some of the orders of the UK market there is no VAT included to the total price and for some the VAT is added (with label "GB VAT 20% (Included)"). We can not find a pattern in the orders.

Common things for both types of orders:

- placed in GBP

- total price of the orders - between 130 and 330 GBP

- the payment gateway is the Default shopify card payment

- user billing address in UK (more specialy - in England) , the shipping address is same as billing

- the products are of type shoes

Here are the Tax settings for the UK market: