This... 5%... at most.
Granted, I haven't played out hours of battles with this (nor do I have hundreds of statistical data points). At best I have anecdotal impressions.
I have played a base defense and had 2 rounds of confrontation with a Spartan Scout in the Air Vents. I'd estimate that 8-12 shots were traded on each side, and cqc failures resulted in 75% of those causing random shots.
I restarted a new campaign last night and fought one distress call. With enemies storming my air bus, I had some of their shots hit, and some appear to miss and hit the door jam. I assumed these were cqc failures on their part. Adversaries were neophyte and church matron. In the Save a Sister mission, my confrontation with the B-boy resulted in 2 rounds of combat. I'd guess 8 shot traded on both sides. 3-4 were cqc misses.
It may be confirmation bias, or extreme outlier cases, but this feels higher than 5% random chance of missing if you are 1 tile adjacent. I thought I'd share my impressions to add to feedback that others are sharing as well.
As it stands, I think players are moving to new tactics to compensate for the current cqc balance. I know I did in the couple hours I've been able to test this. More players are inclined to shoot from 2 tiles away, than from point blank now. This also is my impression.