Hi. I’ve recently noticed this. Strange black guidelines appear whenever I select a point. Please see the samples below.
The first sample is just a screen of point selection mode.
The second is when a point is selected (upper right).
The third is same as second, but a little bit zoomed.
I have seen similar issues before, but it’s not reproducible, so I didn’t even report it. If restarting FontCreator fixes the issue, then I recommend doing that. Before restarting, if you have more than one font open, click the tabs in the order that you want them to appear on restarting.