Fastspring shopping cart is frozen with Brave

I’ve been using Brave on and off for a few months and generally like it, though Safari remains my default choice. On one of my websites I have a popup shopping cart from Fastspring that freezes when used with Brave, but works fine with Safari, Chrome and Firefox etc.

If you go to https://colourmanagementpro.com and click on the add to cart button (either at the top or lower down) the pop up appears, but it is then impossible to fill in details and you cannot even click away from the cart. This happens with or without site shield enabled and the only extension is 1Password.

The website is using Lightspeed and Cloudflare, but I have disabled Rocketloader, since I know that can cause issues with the Fastspring cart. This is Brave 0.23.105, but I’ve noticed the issue for a while now. I’m curious why this isn’t working in Brave, yet appears to be fine on other browsers.