The current behavior is a bit confusing, but actually intended. Disabling the Occludee Static flag does not mean that the object won't be affected by occlusion culling. It just means that occlusion culling won't be performed via Umbra portal queries. When Occludee Static is disabled, the object is still affected by occlusion culling, except it is done by comparing the object's AABB with Umbra occlusion z-buffer instead. All Unity objects follow this behavior, even the ones without any static flag at all as long as occlusion culling was baked.
For MeshRenderers (and anything inheriting from Renderer), it is possible to opt-out from this behaviour by setting the allowOcclusionWhenDynamic property to false here https://docs.unity3d.com/ScriptReference/Renderer-allowOcclusionWhenDynamic.html
For Terrain however, the equivalent does not exist. It should be possible to implement theoretically, but from what I see it looks a bit involved and should count as a feature request.