If Keep Unversioned flag is not set (i.e. the page comes from pre VSN times in the space), and user translates the language and sets the page to be unversioned, the position of the page in the tree will be changed to the alphabetical sorting order.
Steps to reproduce:
Create a space without Scroll Versions or Scroll Translations
Create pages A, B, C, D underneath the homepage - Confluence will order these page alphabetically
Optionally rearrange the order of A, B, C, D to A, C, B, D (actual order does not matter, but it has to be an explicit order - as opposed to the alphabetical order imposed by Confluence by default). If you do not impose an explict ordering the edited page always jumps to the first position.
Create pages 1, 2 underneath A and put on explicit ordering 1, 2 on those pages using view in hierarchy
Activate Version Management by creating a version 1
Switch to Author View and go to page A
Click Edit (you can trigger the same error when you click Translate using Scroll Translations)
In the editor click on 1 (i.e. the working version) and then on Convert to Unversioned Page in the pop-up dialog (or Convert in VSN < 3.2)
Save the page
The pages should stay in the same order A, C, B, D in the tree as before the edit
The pages are ordered C, A, B, D
Go to the page that you want to start editing as an unversioned page and switch to the Public View. Make sure you are on the correct page.
The easiest way to get to the page you want to edit in Public View is by clicking Switch to Public View in the Scroll Versions Menu (see https://help.k15t.com/scroll-versions/latest/write-content-112919353.html). Now open the Confluence tools menu (...) and click on View in Hierarchy
Now click Edit - note that the working version is already set to unversioned. Optionally make changes and then click Save. The page is saved as an unversioned page and will not change position. Any further edit (or translation) will always treat the page as unversioned (regardless if triggered from the Public View or Author View).
If lateron you want to start versioning it use the Scroll Versions Dropdown and click Convert to versioned while in working version context.