When going to punch someone into face noone puts wrist into extended position - slightly flexed is the strongest!
What I mean, let's keep unsafe analogies in our lockers

An issue with fully extended position
might appear because of stretched foreram muscles (superficial flexors group), which may stress elbow tendons and ligaments. You personally play with stiff frames and extremely stiff stringbeds, which makes me conclude you've got
balls elbows of steel and are not in position to judge this part
I advocate avoiding both fully flexed and extended positions, staying around the neutral range - maybe neutral to moderately extended.