Is this screenshot taken in Blender, or some glTF viewer?
The glTF format defines the physical meaning of “transmission” and other PBR properties, but deciding exactly how to implement those — particularly with good rendering performance and framerate — is up to a given 3D renderer to decide. In most realtime renderers today, opaque objects are visible through transmissive materials but other transmissive materials (and perhaps alpha blended materials) are not. Doing more than this can be quite expensive for the renderer, requiring methods like depth peeling, but implementations do exist.
Blender’s implementation of transmission existed before the glTF format added the feature, but it seems to have very similar limits in the Eevee renderer. If I put transmission on two objects the back object remains visible, but once I enable refraction on the back object it disappears when viewed through the front object.
WITHOUT refraction on ground
WITH refraction on ground
To ensure that one transparent object is visible behind another in all 3D renderers, you’ll need to use alpha blending instead of physical transmission. Alpha blending is much cheaper to render, but does not allow glass-like refraction effects.