Either your Workplane or your viewpoint is not properly set to be orthogonal to both views. It may have been at one time, but you adjusted it slightly, however inadvertently.
Choose the Left, Right, World, or whichever view serves your needs for creating the line, then select Plane by Active View and proceed from there.
TC Pro Platinum 2020, 2019, 2018, 2017, 2016 & 2015 (all with LightWorks & RedSDK) & V21
System: i7-5820K @ 3.30GHz, ASRock X99 Extreme4, 16GB DDR4-2133 RAM, Gigabyte GTX 970, Samsung NVMe SSD 950 (256GB), Windows 7 Pro (64-bit) SP1