This is not a mixture of numbers and letters, that is "%lf". VA marks the "lf" as an error. "lf" is the letter "L" plus the letter "F". Generally, "lf" should not be an acceptable spelling, but "%lf" is used for double-precision for scanf() and scanf_s(). Similarly, "%f" is for float.
I'm using VS2019 version 16.1 Preview 3 and Build 2331 of VA, both are the latest versions. I don't think this issue only occurs in specific version. It should be general.
Moreover, it is very strange that VA won't show this error in the spell check dialog.
![](https://forums.wholetomato.com/forum/uploaded/yabowel/201957259_test.png)
But it is certainly marked by VA.
You can use the following code for test:
#include <stdio.h>
#include <math.h>
int main(void) {
double a, b;
printf("Please input a nonnegative number:\n");
scanf_s("%lf", &a);
b = sqrt(a);
printf("Square root is %.3f\n", b);
return 0;
}