Cloudflare will change the nameservers if required for security reasons. If you set the nameservers at the registrar before adding the domain to Cloudflare, that will always result in another set being allocated for example to prevent hijacking.
It is not possible to request to change the nameservers, you will have to set the ones allocated by Cloudflare at your registrar.
If you must have the same nameservers for every domain then you will need a Business Plan and to use Cloudflare Vanity nameservers.
