Some dojo bug in UI likely. There have been a few in Epi 7 related to tiny mce rich text. Try recreate it with a fresh alloy site (v9) and log a bug if you are able to recreate it. If not, then you can solve it by upgrading to newest version. Upgrading after 7.5 is very smooth and you get a whole bunch of similar bug fixes. Epi 7 still have some child sicknesses after completely changing UI for editors. Version 9 is much more stable and polished so I would definitely recommend it either way. It will probably take longer to fix this one bug in 7.5 than upgrading to 9. Definitely an epi bug so nothing you can really do except report and then update when it's fixed. Informing your editors and use work around also works of course but it's annoying.
I use CMS 7.5. I edit a field (example: Main Body) in On-Page Editor (OPE) to "Abc", autosave successfull. Then, I leave it about over 5 minutes (browser inactive 5 minutes). Then, I come back, continue edit it: "Abc" => "AbcD", autosave successfull. Change to "All Properties", not do anything, then, back to OPE. "Main Body" field show "AbcD", but when I click it to edit, it show "Abc", see 2 images (after from "All Properties" back to OPE). When I try to refresh browser, I see data is saved, everything is normally.
How I can fix it ???