from what I have experienced- blocking their first attack will not put you -in combat-
Not until they get a successful or near success(a swing and a miss) will it put you into combat for you to have an auto target.
Later blocks shouldn't drop you out though.
Usually my first two attacks are targeted and that generally covers any initial blocks so I can then just cast globe/pray strike/k/etc.
If the enemy does get the first swing you are usually good to go though.
I don't know if that was intended or not either, but if you are just trying to move through a low level area quickly it can keep you out of combat status.