Incorrect currency conversion in WooPayments

I just put my site live after migrating from a different cart system. I have WooPayments Multi-currency enabled and expected it to automatically convert GBP amounts to USD and EUR, but today I got my first order and the customer paid $27.26 USD for an order that should have cost £27.26. Same amount, different currency, no conversion. The exchange rate is currently 1 GBP → 1.32 USD, so why did conversion not take place as expected?
I just disabled “Automatically switch customers to their local currency if it has been enabled” in case that had something to do with it, but somehow I don’t think that’s the problem.
I checked the transaction details and I see:

A payment of $27.26 USD was successfully charged.

1.00 USD → 0.758254 GBP: £20.67 GBPFee (5.5% + $0.33): -£1.39

• Base fee: 1.5% + £0.25
• International card fee: 2%
• Foreign exchange fee: 2%

Net deposit: £19.28 GBP

So the store is sending the wrong amount to the payment gateway, it’s not being converted at source as expected.

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