A quick look indicates that the I beam was not placed orthogonally on your workplane, so when you split it, its long dimensions were at a slight angle from the Z axis.
Try taking your original piece and placing one of the long facets on the World workplane (to do this, go to View/ World Plan, Plane by Active View, draw a Box with negative height (so its top facet will be on the World workplane), then use the Assemble By Facet function on the I beam to place it on the Box's top facet.
Once done, 3D Slice and Mirror to see how it turns out.
TC Pro Platinum 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