Before freaking out there needs to be several things done.
First: Reproduce it. Go into a custom game, buy sorc boots with home guard and show the stat. Gonna go out and say it, pics (vid) or it didn't happen.
Second: Make sure it's not just a display error and get a friend to be punching bag. Prove that the extra spell pen is actually being calculated into damage. Either way it'll be a bug but it will drastically change the priority of the bug fix depending in the result.
Third: Submit an abusable bug like this (after confirmation) to the support team via a private ticket so as to NOT advertise it. It makes it much easier on Riot's end if it can be fixed out of the limelight, and it severely limits a red's capacity to respond to a thread like this as a red comment brings in a lot of visibility.
Personally I would do this myself but I'm stuck in my undergraduate research lab waiting for 3d model renders.....