Sigma 50-100 1.8 hunting @ 100mm

SIGMA 50-100mm f/1.8 DG HSM 'Art'
Paired with Canon EOS-1D Mark IV.

Please no questions about using a cropped lens on an APS-H camera, yes this combo is confirmed to work well together, that is not the problem.

Autofocus thru the viewfinder (Phase-detect AF) works fine at 50-60mm, but becomes increasingly unsure as you zoom, becoming impossible at 85-100mm.

Interestingly, autofocus works perfectly throughout the entire zoom range when using Live View (Contrast-based AF).

The lens is in mint condition, and I do not know what could be causing this problem.
I have a Sigma lens dock in the mail to preform a firmware update.
This happens on both my 1D IV and Rebel G film SLR.

Would a problem like this still happen on mirrorless? Since all AF processing is on-sensor?