UK VAT is deducted sometimes but not others

[skyesilver]

UK-based store, VAT registered. All tax settings are set correctly:

"Charge tax on this product" is ticked in the Product editor
"All prices include tax" is ticked in Settings > Taxes
"Include or exclude tax based on your customer's country" is ticked in Settings > Taxes

If a non-UK customer (that is, a customer with a shipping address outside of the UK) places an order, the UK VAT of 20% should be deducted. For example, if an item costs £74 including VAT, a customer from say Canada should only be charged £61.67. This has been working perfectly up until last week or so. Since then, we have had several non-UK orders where VAT has NOT been deducted. However, the problem is not repeatable. For example, there are two orders today from Canada, both for exactly the same item, yet one was charged VAT and the other wasn't.

Has anybody else had this problem, or more generally problems with how Shopify deals with United Kingdom VAT? Shopify themselves seem clueless about tax matters outside of Canada/USA, based on the discussions I've had in the chat with them.

[Xtma]

Hi Skyesilver! 
May I ask if you found a solution? The 20% automatic deduction at checkout for international customers, is this a Shopify Advanced feature? I was just told by a Shopify Support person that this feature does not exist on any Shopify plan, and he said it was confirmed by their tax team (!)

Yet I see it all over Shopify stores and you mention it too. 

best from desperate UK VAT registered person on the seventh or so hour of "Shopify Support" : )