First off, underpaying is also a bad problem. Secondly, if those 14% are the most expensive ones, (hint: they mostly are) that's way too high of a number. Third, it completely depends on insurance. ACA's study showed it was completely dependent on insurance, with some insurances having as high as 45% claim denial and some having as low as 1% claim denial. Taking insurance across the board when we are talking specifically about poor people getting their insurance denied- who by definition almost certainly have low-end insurance- the claim of 14% is misleading at best.
What you are looking at, I assume, is the American Medical Associations number. But that number has long been disputed by healthcare workers, because it only looks at at all itemized lines that are denied. It does not consider the type of claim that was denied. It also does not consider how long you had to wait to get your claim approved, its merely a blanket look at it.
In 2017, the Doctor-Patient Rights Project found a denial rate of 24% among people with insurance who had chronic or persistent conditions- the ones who would be most affected by denials, since they are in constant need of medication. In addition, the vast majority of consumers- 70%- said they had to wait more than a month to even hear whether or not their claim was denied or approved, during which time a third of them said their condition worsened.