Steps to reproduce:

1) Download attached project 'Texture2DAlphaIssue.zip' and open in Unity
2) Build '0' scene
3) Download Renderdoc and open it
You can download Renderdoc from here: https://renderdoc.org/
4) In Renderdoc: Capture Executable >> launch .exe file of built project in Renderdoc
5) Press F12 to capture the frame
6) Double click on capture so Renderedoc will collect the log
7) In Event browser, expand Color Pass #1(1Targets +Depth) and select DrawIndex(6)
8) Select Texture Viewer tab, with r.mouse click on the red circle
Note that the value of alpha will be 0.24706 even shader written in Unity should make this value to 0.5 (check shader 'TextureShow.shader' in Unity)

Expected result: The value of alpha should be 0.5
Actual result: The value of alpha is 0.24706

Reproduced with:
5.6.3f1, 2017.1.0p2, 2017.2.0b5, 2017.3.0a2

I wasn't able to check with 5.5.x because shader in this version didn't worked correctly

Note: Video of reproduction is attached (repro.mp4)