Sometimes when marking the first portal location with and then hitting E to place the second, E will instead place both locations, one at Medivh's feet and one at target location just like normal. However the first location you marked with Portal Mastery will stay marked, and as soon as Medivh's "normal" portals expire you it can instantly recast E and link with the old marked location at no cooldown, essentially allowing you to have 100% uptime portals again. At least in a small area.

This can be incredibly annoying and incredibly powerful in certain situations. It happened to me a few times over two games(On punishers and Braxis in case it matters) and I was unable to replicate it in test mode, though I think it may have something with being over terrain when you mark the first location.

Let me know if you can replicate the bug somehow, or if this is already a known bug. Hope this helps, HOTS community! GLHF