This is strange.
I tried Chrome (with all extensions off), Firefox, and Edge. Still the same problem.
I tried re-downloading the files, making sure I was not using v2 or the v3 betas.
I tried with just local files, as well as on a local server. (just checking off boxes).
I tried removing all code links in your pen, except gsap.min.js and GSDevTools3.min.js to make sure there were not any dependencies I had not included.
In the codepen you made, the dragging works on the playhead and both in and out markers (in all my browsers), but there is still another glitch I had not noticed before.
Once you reach the end of the timeline, if you try to use the rewind button, playing from the start fails. It jumps right to the end. Dragging the playhead to the start allows you to replay, but not the rewind button. This behavior is the similar if you move the in/out markers. If you move the out marker, the playhead makes the same "jump to end" behavior; but if you move the in marker even the smallest amount, the timeline works as it is supposed to.
Side note: The javascript in the codepens, in the v3 documentation, is still using v2 code.