@oceane If you're not on GitHub then I can help here 🙂
Reader Mode is purely visual; it has no effect on saving.
How are you opening the file? If you aren't choosing the file from within Orgro (e.g. opening from another app via an intent) then Orgro knows it can't save changes.
When Orgro knows it can save changes it will ask for permission to save with this banner. Did you see this? If you've answered Never in the past, you can reset with ⋮ > Clear cache from the top screen.