There was nothing wrong with the M16/AR-15; the Marines had been issued the weapon in Vietnam as well (with different ammo than the army received) and it worked fine.
The issue was that the Army Bureau of Ordinance insisted on making 5.56mm ammunition with a propellant composition different from the one that Stoner had specified when designing the weapon, one that was entirely unsuitable and led to jamming.
Yes but it was worse due to design problems with the gun as well, seeing as how they did change it - I.e. adding chrome plating to the chamber and barrel which reduced fouling, and actually including a proper cleaning kit.
The chrome lining was done to significantly increase the service life of the barrel and to reduce corrosion in some environments. Prior to the M16, chrome lining was only used on machine guns due to the volume of rounds that went through them.
Far more rounds were put through the M16 by soldiers than prior weapons like the M14. Despite the chrome lining, M16 barrels still wear out over time and have to be replaced.
"in some environments" being the Vietnam war the US was involved in, in reaction to casualty reports where a large number of men were killed with their rifles broken down for repair likely due to mid battle stoppages.
And the same applies to number of rounds: the entire point of a 5.56mm cartridge was to give soldiers more ammo so they could sustain volume of fire in the field: it was a design goal.
If design changes fixed a serious flaw with a weapon, then it was a flawed design.
The issue was that the Army Bureau of Ordinance insisted on making 5.56mm ammunition with a propellant composition different from the one that Stoner had specified when designing the weapon, one that was entirely unsuitable and led to jamming.