Tax not applied when changing / upgrading membership

Using MoneySuite and Stripe. Here is an example. There are three memberships priced at 14.99, 24.99 and 49.99 and tax is installed at 20%

User signs up to the £14.99 package and is billed £17.99 per month as expected. They then go to memberships and select to change their membership. They have the other two more expensive options available. The user selects the £24.99 option and TWO things happen which are incorrect:

The website tells them they are going to pay £10 (which is technically correct for this upgrade as they pay the difference on the first one which is £24.99 - £14.99 = £10) BUT actually charges them £24.99 which is an over charge of £14.99 AND

The website sets up a recurring charge of £24.99 which should be £29.99 per month when you add on the VAT