Wall section closing on itself when exporting to obj

Hi guys, i need help on a little thing i cannot figure out: i have the wall showing in the images i have added in wich i have opened a doorway, now when i export it to obj that section closes itself so i have the result showing in the second one ( it is the model as it appears on keyshot renderer); i’ve only managed to notice that the part that closes corresponds at the wood material i have assigned.
How can i fix this?



Check for any faces or other object you have hidden from your d view (Alt+H to unhide)
If you wall face is made up of ngons (faces > 4 sides), make them into tris / quads
Go and actually look at your face for any other errors