FYI, below is the response I got from PayPal Merchant Technical Support. In short, this is definitely a PayPal issue which they will try to rectify, but the interim workaround is to set Auto-Return to "On", which I don't think I can do as I'm selling a downloadable product. They did send me another message advising they will notify me when its fixed however, so I'll update this post when that happens. Cheers.
Thank you for contacting PayPal Merchant Technical Services.
I've made a testpayment for 0.01 and i could reproduce the issue. We will reverse that transaction as it was a test. However, it appears that this is only happeing if auto return is turned off. You can turn it on and then the amount should show.
Login to your PayPal account
click on Profile
click 'My selling preferences
click on Update next to Website preferences
set Auto Return to On
enter a valid URL in the Return URL text box
scroll to the bottom of the page and click "Save"
I can confirm that this is an issue within our system and i've escalated this to our developers which will fix that. However, as a workaround you can turn on auto return.
I hope this was helpful and if you have any further Questions, please let me know.