[bagptarmigan]

Hi,
     I don't know the Canon 80D camera, I shoot Nikon, however the symptoms you are describing of general strange erratic AF behaviour are similar to the issues I have had with Nikon DSLRs ..... often it's dust on the AF detector prisms under the mirror, remove the lens ..... (with the Nikon I go to clean sensor/raise mirror or the Canon equivalent in the menu) and then with the mirror in the raised position, use a rocket blower to blow off any dust which may have settled on the AF prisms under the mirror, it's a 5 minuet job which usually does the trick .  
    If you haven't a rocket blower they are cheap as chips on ebay and are very useful.

pizza tower

  Certainly that would be my first port of call.
good luck ......

Thanks for the tip! I've been having similar issues with my Canon 80D, and I never considered the possibility of dust on the AF detector prisms. I’ll definitely try using a rocket blower as you suggested. Hopefully, this will solve the problem. Appreciate the advice—fingers crossed it works!