Hey, thanks for the report. This is a confirmed keybinding conflict.
We had a similar case before where Cmd+Shift+S got overridden by the agent sidebar (see the related thread: Cmd + M on Mac (minimize) no longer works due to default keybinding for `workbench.action.showAllEditors`).
Workaround: Open Keyboard Shortcuts Cmd+K, Cmd+S, search for Option+Cmd+S, right-click the toggleUnifiedSidebar entry, and remove the keybinding. This should restore saveAll behavior on that shortcut.