I know this may not be very helpful, but I’m currently spending some time adjusting velocity values and haven’t encountered the issues you describe, at least not when changing the values in this window:
I have my main window open in Write mode (Galley view) and a second window in Play mode, and everything behaves as expected. I’m using version 6.1.1, but I didn’t experience any issues with velocity editing in version 6.1 either.
If you haven’t yet explored the Transform tool, you may find it a very useful addition to your toolkit when adjusting values across a range of notes in the Key Editor.