As BobT noted, you are getting light leakage from behind the filter.
Lee's "Stopper" line of filters have a foam gasket on the back side of the filter, on all edges. For extreme filters such as the ND4.5 / 15-stop Super Stopper, it's absolutely imperative to block all light leaks.
Looking at your Zomei filter holder, there is no single flat surface where a square foam gasket will completely block all extraneous light from behind:
- The "scallops" at the top left and right will likely cut the corners behind the gasket.
- It looks like it might be possible for light to leak behind the filter at the very top and bottom because the curvature of the filter holder might be larger than the dimension of the filter.
- The adapter ring surface does not sit flush with the front face of the filter holder. If it were to sit flush, as the Lee filter holder does, it would provide additional gasket bearing surface on the tops and sides of the filter, to help block more light.
I believe your Zomei filter holder is not up to par for light stopping filters such as the Little/Big/Super Stoppers. It's probably fine for 1–4 stop ND or ND grad filters, etc. (which don't have light-blocking gaskets), but not for any gasketed filter.
To make up for the light leaking on the Zomei, you will need to be diligent to completely block the light from behind with light-blocking material, gaffer's tape, etc.