This general bug has an open Github issue: Panel width for plugins do not persist · Issue #5634 · laurent22/joplin · GitHub
For now, here's a workaround that doesn't require custom CSS. Move any plugin you want to preserve the panel width of left of the editor, as described and shown in this Github issue: Outline width is reset after toggling off and on · Issue #33 · cqroot/joplin-outline · GitHub
For you specifically, comment out the above Note Tabs plugin CSS and restart Joplin before moving plugins left of the editor. Not doing so can create unexpected whitespace when moving plugins. You can uncomment it after plugins are moved.