Removing the mixin is removing the gltf-model component but you don't see it disappears in the right panel (a forceUpdate is missing on the ComponentsContainer React component for the entityupdate event with component "mixin"), you can select another entity and select the previous entity back to see it.
Another issue is the UI is not updating with the mixin name at the top of the right panel and in the left panel when you add or remove a mixin.
When you add a mixin, a gltf-model component is added but the UI is not updated.
I mentioned the issues in https://github.com/aframevr/aframe-inspector/issues/689#issuecomment-1646793789 where we have mixins setting a gltf-model component.
Removing the mixin is removing the gltf-model component but you don't see it disappears in the right panel (a forceUpdate is missing on the ComponentsContainer React component for the entityupdate event with component "mixin"), you can select another entity and select the previous entity back to see it. Another issue is the UI is not updating with the mixin name at the top of the right panel and in the left panel when you add or remove a mixin. When you add a mixin, a gltf-model component is added but the UI is not updated.