Star shapes on Samyang 135mm

There’s a couple things going on here. The EOS 6D has 6.5 μm pixels. The pixels of the IMX 571 are about 1.7x smaller, so any issues are going to be much more apparent when using the 571. Another problem is that the lens was manufactured for use with a Canon DSLR, so the corners are going to look better out of your 6D than in an astrocamera. 

These two issues notwithstanding, it’s really hard to tell if you have sensor tilt or if the lens is decentered unless you attempt tilt correction with a good device like a Photon Cage. Only by trying to adjust the problem corner can you tell if that corner is being tilted or if there’s something inherently wrong with the lens. 

I will say that my Rokinon 135 shows issues with APS-C and larger, even when stepped down. My Sigma 135 behaves a lot better.