We are unable to use texture packs on a realm.
Several texture packs are affected by this issue.
However the pack is well displayed in the realm settings.

I did tests with the modern HD pack and different packs offered in the realm subscription

update : It’s very simple to reproduce: just put the Modern HD texture pack, for example, on a realm, and launch the realm. There is no error message, just the texture pack that is not taken into account. Nothing complicated to reproduce