Is it an issue with the monitor? Does it show up in the bios screen, or on the monitor splash screen
Does your board have video outputs? If so, try removing 1070 Ti and see if line goes away. An issue like that is either cable, monitor, or GPU.
Seems like it's going to be a monitor issue then. Different cables and completely different GPUs, both discrete and onboard, have the same purple line. Are they any adjustments for monitor itself, such as a reset, or changing resolution, ect.