The QActions in the menu bar don't work when the menu bar is hidden, which is the default state. Thus, saving via CTRL-S is not possible without first hitting ALT. Some discussion of the issue: https://forum.qt.io/topic/15107/solved-action-shortcut-not-triggering-unless-action-is-placed-in-a-toolbar
Fixed in b4543062e0c62f893861cc41067f93e13a588ce8, thanks!