the dynamic cubemap used as envmap is circled. It should not be restricted to the small square area but should cover the complete, striped sphere. Setting transmission to 0 shows the correct mapping for the cubemap. Perhaps the cubemap is using the same UVs as the texture internally used for the transmission if there is transmission ?
Well, the cube camera does work correctly if transmission is zero even when placed inside. Also, it works correctly in the dynamic cube map example with TwoPassDoubleSide where it is also inside the sphere. So there should be something else which conflicts with transmission.
But it makes sense that a rendered inside would occlude everything else. Conceptually, it would make sense to set the position of the camera far away from the origin, in the reverse look at direction, for each side. This seems like something which probably was discussed a long time ago, and discounted. Perhaps because it is difficult to know what ‘far away’ means.