I had hoped the new version of Audacity (v3.3.2) would have stabilized the very useful Envelope Tool. But if the Amplify tool is used more than once to edit part of a track before using the Envelope tool, the control points still jump to the beginning or an earlier time. It seems the Amplify tool negatively affects the Envelope tool.
Exporting the problematic track and then importing it again restores the ability to use the Envelope Tool anywhere in the audio clip…until the Amplify tool is used again. Is there a setting in Preferences that might stabilize the Envelope tool?