Currency symbol and amount not matching on Cart page

Hi team you can see the issue in the link we provided.

Our set-up: We have a few viewing currencies but only allow customers to check out in USD

The issue write-up:
1. Customer adds a product costing SGD $27 into their cart
2. Cart symbol on top right hand corner showing correctly SGD $29 (including tax)
3. Customer goes to cart page but (i) cart page shows USD $28.91 but this is wrong – it should be showing SGD $28.91 and (ii) if you look at the cart symbol on the top right hand corner it is now showing USD $29 which is wrong – it is supposed to be SGD $29
4. Customer goes to checkout page and the USD amount is correctly showing as USD $21.40

Can you please let us know how to fix this issue?