Steps to reproduce:
1) (We've figured out how to reproduce this everywhere.)
2) Open a page in the visual editor. (Therefore, you are editing the whole page, **not** a section.)
3) Make a change. **Use the "Edit Source" tab at the top of the page to** switch to the wikitext editor. Choose "Keep changes".
4) Make a change. **Use the "Edit" tab at the top of the page to** switch to the visual editor. Choose "Keep changes".
5) Make a change. **Use the "Edit Source" tab at the top of the page to** switch to the wikitext editor. Choose "Keep changes".
6) Discover that all of your changes were lost – this time, but not any of the previous times.
This is consistently reproducible in 100% of attempts by following this exact process, both while logged in and while logged out/using a private browsing window, in miltiple browsers and systems, but **only if you use the tabs at the top of the page** instead of the buttons in the toolbar.