I'm having the same exact issue. My two canon lenses don't have this issue at all. I have two T8i's in front of me and it's happening to both. One is used and one is new. I tried cleaning the contacts just to be sure. The funny thing is that on my T7 I have zero issues. Anti Flicker is off. I also noticed that on live view I can only do manual focus, but it keeps flickering like it's trying to go to auto.
Update: Okay so I played around some more and I found that if I unscrew the lens a millimeter or two counter clockwise the issue doesn't happen anymore but I still can't get auto focus in live view though which is really weird. Clearly there's an issue with the contact points lining up on a T8i, though I don't know of any stated difference between the two.
Update again: Doing MORE digging it turns out there's a firmware update to address this exact issue, though it mentions the T6i, not the T8i. I'm sure it's the same thing though. I will call Tamron and see what they have to say.
FIX: Okay not so much a fix as a work around, but in the camera settings, camera tab, menu 1, go to "lens aberration correction" and turn off the optimizer. Once you do that, new options show up and turn off "chromatic aberration correction". That fixes it. I still have to do MF on live view, but that's good enough for me. If i'm doing live view I have time anyways. PHEW. Please see relevant link:
https://www.dpreview.com/forums/thread/4273673
