The Shopify Dev team is aware of this bug, but since it may be affecting other users without their knowledge and we’re headed into the holiday shopping season, I wanted to detail it here as well. If it is affecting you, it may be worth pointing it out to support so they know how prevalent the problem is.
If you use custom blocks to render an image in checkout, it may not work on mobile the first time the page loads (at least on iOS, not sure about Android). If other page elements are cached, it will load. To test this, you need only to open your website in an incognito/private window on mobile, add to the cart, and go to the checkout page to see if the image is displaying.
Keep in mind that this only occurs the first time the checkout page is visited. If you back out of checkout and go back in with the same incognito window, or if you refresh, it will load. If you don’t use a private/incognito window and you’ve visited your checkout page before, it will also load. We added an image specifically to improve conversion for a client, so it could potentially affect overall sales during Q4 if it isn’t fixed quickly. The issue does not occur on desktop.
I am not sure if this is affecting implementations of custom blocks on mobile besides the rendering of an image, but if you do have any custom blocks in checkout, I would recommend testing with an incognito/private window to ensure they’re showing up.