I believe the FoV is based on aspect ratio rather than explicitly horizontal or vertical. See the following sources, some of which offer ways to deal with or get around this, if needed:
Foldable Field of View Guide – Unreal Engine | Samsung Developer
Sathvick Views: 10 D02 H01 P02 Focal Length and Field of View Settings in Unreal Engine 5.6
For good measure:
Cameras in Unreal Engine | Unreal Engine 5.7 Documentation | Epic Developer Community