Contacted LG again and they are still waiting for the head office to come up with a solution.
As I was reaching the deadline to return the LG 29UM67 I did just that and I've got a 34UM67 instead.
I've bought it from Amazon.it a couple of days ago, as it was at a decent price at the time (Euro 449,99). Received it today, just gave it a test run and everything looks good.
No gray bar anymore and another problem as I was having with the 29UM67 seems fixed as well. I am using two inputs (DisplayPort with the Freesync PC and DVI with a second machine without Freesync) and when the monitor was going to sleep while on the second input it would lose Freesync on the first input, so, when coming back from sleep, I would have to manually turn Freesync off and on again for it to work.
Both these problems seem to be gone with this machine. For reference I have a R9 290.
Edit: Upon further testing the second problem is still there, but the gray bar is definitely gone.