Hi there Bastien!
Don here from Shopify!
While I can't be 100% sure without looking into your store specifically, it looks like the system has defaulted to interpreting this tracking number as belonging to the Royal Mail carrier.
When you add a tracking number to an order in Shopify the system will try to automatically detect which carrier this pertains to and will select the closest match based on know formats for carriers.
This is less than 100% perfect, as it can sometimes incorrectly default to an incorrect carrier. We're working on improving the accuracy of this all the time; though it is a work in progress.
What you can do, however, is to just change the carrier in the order itself from 'Royal Mail' in this example to 'Other' or the carrier of your choice if it's one of the defaults there.
This should remove the automatic link to Royal Mail and alleviate some potential confusion for your customers. :)
If there's anything else we can help out with, feel free to get in touch via our support site.
All the best!