Well, I have now re-purchased the 29UM67-P and have it on my desk at the office. I can't test freesync right now as nothing here even has displayport let alone an AMD gpu.
But, the manufacturing date is December 2015 so it shouldn't have the grey line issue. My old model would do freesync all the way down to 30hz which also enabled the AMD low frame rate compensation. Hopefully this monitor will OC to 80hz and I'll be able to get freesync working between 30hz and 80hz! I'll let you know how I get on.
For what it's worth, it seems that LG have discontinued these monitors now. So if you're in the market for one, buy now.