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Update 2:

Writing v_texCoords.x to the separate float and using it in mix() fixes this bug but only for desktop when #version 100 is also used:

#version 100
uniform sampler2D u_texture;
varying vec4 v_color;
varying vec2 v_texCoords;
uniform float u_smoothing;

void main() {
  float x = v_texCoords.x; // Writing to the separate float
  
  // SDF
  float distance = texture2D(u_texture, v_texCoords).a;
  float alpha = smoothstep(0.5 - u_smoothing, 0.5 + u_smoothing, distance);
  vec4 sdfColor = vec4(v_color.rgb, alpha * v_color.a);

  // Regular
  vec4 regularColor = v_color * texture2D(u_texture, v_texCoords);

  // Making a choice
  gl_FragColor = mix(regularColor, sdfColor, step(1.0, x)); // Using x instead of v_texCoords.x
}

It doesn't fix the problem on Moto G. Also when I move float x = v_texCoords.x; to the line right before gl_FragColor = ... the problem remains on desktop:

void main() {
  // SDF
  float distance = texture2D(u_texture, v_texCoords).a;
  float alpha = smoothstep(0.5 - u_smoothing, 0.5 + u_smoothing, distance);
  vec4 sdfColor = vec4(v_color.rgb, alpha * v_color.a);

  // Regular
  vec4 regularColor = v_color * texture2D(u_texture, v_texCoords);

  // Making a choice
  float x = v_texCoords.x; // Writing to the separate float, this time it doesn't fix the bug
  gl_FragColor = mix(regularColor, sdfColor, step(1.0, x)); // Using x instead of v_texCoords.x
}

Update 1:

Using different precision qualifier (highp / lowp) or completely removing it didn't help and made no visual changes.

This bug starts to appear on desktop (OpenGL) if I add #version 100. Changing it to #version 110 or higher returns normal rendering.

When #version 100 is used on Android (OpenGL ES 2.0) it renders as if there were no #version qualifier at all (exactly the same as described in my question - on Nexus 5x it looks good, on Moto G it doesn't).

Using #version 300 es on Android (OpenGL ES 3.0, looks like it is supported Moto G 3rd) with respective changes to the code (uniform, varying, texture2D replaced by in, out, texture) doesn't fix the problem (good on Nexus, bad on Moto).

Precision qualifier was not used in both cases. Looks like #version does no effect to this problem on Android but makes it appear on desktop when set to 100 (is it OpenGL 1.0?...)