Problem of difference between R16F and R32F in shader

OpenGL
1

.cpp

I stored TEX and texture for fbo with format R16F .

.frag

uniform sampler2D TEX;
uniform float uniformValue;

case 1 : load uniformValue -> 0.0f

{
  float texColor_R = texture(TEX, texCoords).r;
  float mixFactor = uniformValue;
  float R = mix(0,texColor_R,mixFactor );
  gl_FragColor = vec4( R,R,R,1.0 );
}

case 2 : fixed value(0.0f) in code

{
  float texColor_R = texture(TEX, texCoords).r;
  float mixFactor = 0.0f;//@change
  float R = mix(0, texColor_R, mixFactor );
  gl_FragColor = vec4( R,R,R,1.0 );
}

case 2 works fine but case 1 does not.

I did not know what the problem was, so I just changed textures(TEX and texture for fbo) to R32F and it worked fine at both case1 and case2.
Why does this happen?
please let me know how to avoid the texture formatting issue.

2

Define “works fine” and how the other one does not “work fine”.