I would think we would be testing the ever-loving shit out of our new toys. My takeaway from that episode is that they didn't care enough to fix the flaw because anybody willing to do sick tests on people isn't going to be overly concerned about the ethical consequences of allowing such a weakness in their armor.
I do agree that it's highly unlikely anybody outside of us will ever know about such a flaw but if we are dropping T-60s onto the battlefield sooner or later somebody will get a lucky shot and drop one in one shot. Word gets out and more aim for that spot and drop more. A generational long war allows for more than enough time for somebody to learn about it even if by sheer dumb luck.
Thing is even a lucky shot isn't going to be enough you're going to need armor piercing and hit a VERY small section in addition to have a gun that already has a significant amount of KE
Remember in the observatory prior to the Ghoul fight the only Knights that were killed were by explosives or stuffing a gun barrel down the neck gap for the armor and holding the trigger, the NCR had tons of guys mass firing at power armor with little effect
FN Fals did nothing in mass numbers
M1A1 rifles did nothing in mass numbers
M14's did nothing in mass numbers
A SHIT Ton of M1918's in mass numbers did nothing as well
The odds of a lucky hit going through would require precision, power and armor piercing rounds of something like a sniper rifle which is unlikely to hit the area because it's VERY small not visible, small enough that we don't even see the hole from the killed power armor trooper, it's still a flaw but it won't be found out by all before the Government fixes it tbh.
FO America didn't fix it because it's supposed to be mass produced cheap armor