Saving after Edit PDF ruined all unedited text (bilingual document)

Hi all, hope you will be able to save my day's worth of work.

I have been editing musical sheet in Acrobat Pro. Changing the text from English into Russian.

After saving the document (Cmd+S) all the unedited text got messed up and changed. Like some insane encoding switch.

My guess is that at some point the document was scanned for reading in Russian, but not English and that caused a mess-up.

This is not the first time I am having this bug. Right after saving the file.

Here are the before and after samples: