I know how sketchy it sounds, and it was even documented in DB Navigator. But DB makes this information inaccessible a couple of days later in the app, so there is no way to „proof“ it except asking DB itself.
Is this the right connection?:
https://bahn.expert/details/NX%2028521/j/20251224-a0049123-9...
The RE5 seems to be operated by National Express, not Deutsche Bahn, right? (but DB InfraGo is most probably responsible for the routing)
It is operated by National Express, but I guess the routing comes from some other company (likely InfraGo)