If there is a sizeable gap bigger than 3mm along the edges of your TV stand doors, the culprit is most likely the skinny boards that sit on both sides of the fireplace insert, parts "H". To remedy this issue, the unit would need to be partially disassembled followed by the parts being properly attached.
As shown above, the red arrows exhibit that both "H" parts should be facing outward.
This adjustment may seem minimal, but it makes a world of a difference by diminishing those undesired gaps.