Minimap blacks out in mines

Whenever I enter a mine or cave tunnels, the minimap blacks out. The arrow cursor still shows, but the map is black. If I use the mouse wheel inside the minimap to zoom in or out, the picture returns (showing the tunnels, etc.). But as soon as I start moving, it goes black again. Known issue? Or any fixes?

/reload ui

maybe that will help

Thank you. Turns out it was a default setting in the BlizzardUI Tweak addon.