This seems like normal behaviour, and should be affecting all types of animation, not just shape keys.

Since you already keyframed that value, any changes you make to that value are, logically, overwritten by that keyframe when the timeline moves, unless you say otherwise, by placing a new keyframe.

If you find this unintuitive for your workflow, take a look at the Auto-Keying option. This will automatically create a keyframe for you when you move something(including shape key values). It can be quite dangerous if you forget about it and unwittingly move things around without meaning to put down a keyframe.

Hope that helps, let me know if I misunderstood the issue.

Thanks for the response but I don't think that's what's happening here. Coming from video, I'm familiar with keyframes and auto-keying, and this is odd behavior. I'm saying when I try to adjust the value at any point in the timeline after the first keyframe is set, the value will snap back immediately. Blender will not let me adjust the value at all. It doesn't matter if I set the second keyframe before or after adjusting the shape key value, or whether or not I use auto-keying. The value will not change.

Oddly enough I found one exception. I had an audio track in the video sequencer so I could time the animation to the audio. When I deleted the audio and tried to make a new keyframe with a different value, Blender let me make the adjustment. But now it's back to how it was before, snapping back to whatever value it's currently set to.

Sure. I simplified the project a bit and just left the object I'm having issues with. Gave it another test and it's still coming up with the same problem.

I tested the T60598 file you attached and that's working perfectly.

Okay, so i found a way around this bug, as one of my animation was having issues with the same problem you are facing.

How i got around it was i selected all of my character, ctrl - c and opened up a older version of blender like 2.79
i pasted my character in there and adjusted the shape keys there, as well as finished my animation there as it is more stable currently.
after i was finished i did the same ctrl - c after selecting my character and deleted the one in 2.8 and pasted the one i now had in 2.79 with the changed shape keys.

while its a hassle and i hope this bug is fixed, it was the only way and answer i found on this particular issue i think you are currently going through.

Hey, in case anyone's still having this issue, I've got a solution of my own.
So the slider doesn't appear to be working, but fortunately you can type into the slider.
Well, it still doesn't work in most cases, but if you use the Auto-key thing, it'll work.
So yeah, just turn on the auto thing, then type the values of your stuff, make sure you do actually click the keyframe button next to the slider though, and it should work, worked for me.
Only problem is that if you want a precise value for your shape key, you're gonna have to go through a bunch of numbers.
Hope this helped!

I just have find out where the problem is in my case.
If I set my scene>dimensions>time remapping back to : old 100 new 100 (befor it was old 22 new 100)
and it works well.