what does this graphic mean?

[ComradeP]

It's most certainly not a graphics glitch.

It means the minefield or obstacle hex is Penetrated.

 Obstacles are considered Penetrated after any unit enters them and from that point on, their cost depends on the status of the entering unit: • It costs Deployed units an additional ¼ of their full movement allowance to enter Penetrated Obstacle hexes. • Units in Travel Mode have no additional cost to enter Penetrated Obstacle hexes, but cannot do so using Road Movement.

By default, a minefield blocks Supply through that hex.  After a unit enters a minefield, it is considered Penetrated.  A Penetrated minefield does not block Supply.

Penetrated obstacle or minefield hexes have the word MINES or OBSTACLES displayed in lower case in the hex information area. Obstacles and minefields are considered penetrated after any unit moves into the hex.