My issue happens inside ADMIN, and not in the frontend submission plugin.
Anyway, the problem seems related...
Same problem here!
I have a MB SETTINGS PANEL with tabs, inside the tabs I have columns, and inside the columns some color configuration metaboxes.
When I try to open one of them, I have a problem of overflow hidden and z-index.
If I modify the CSS of the color picker from ABSOLUTE to RELATIVE, it works (adding a traditional effect of folding open/close to the box).
I have tried the color metabox WITHOUT placing in columns, and works fine.
If I put 2 of them side by side, the problem returns.