Some devices and QR Code readers display the full URL or the domain encoded on the QR Code before accessing it. The purpose is to be clear and to know upfront where are you going to go if you scan that QR Code. If you create a dynamic QR Code in a paid account, a short URL containing our domain and a subdomain you have selected, e.g. https://yoursubdomain.qrplanet.com/xxxxx, is encoded in the QR Code.
The subdomain is too long
In some cases, including iPhone devices, the subdomain is trimmed because it is too long to be displayed in the preview and therefore only the root domain will be shown. This usually happens with domains like toolongsubdomain.qrplanet.com which will be trimmed to qrplanet.com for example on an iPhone.
How long can a subdomain be to be displayed?
There is no straight answer to this. It will depend on the size of the device, the App that you use, the width of the characters of that specific subdomain and how Apple decides to display the preview in the future.
Use a custom domain
If you want to make sure that qrplanet.com does not show on your domain and you want to have your domain displayed when a QR Code is scanned, you need to go for a White Label Plan.
iPhone scanning a QR Code with custom domain