PL/SQL Developer 14.0.6.1988 saves the sql, prc, ... files in unicode format, but unicode is not enabled in preferences.
It is a problem for us, because we have to use SQL*Plus, and other text editors for other tasks, which does not handle unicode.
The problem can be reproduced.
When I write the text directly into PL/SQL Developer, it does not uses unicode.
When I copy a text from this old text editor into a pl/SQL Developer program editor, and save it, it changes to unicode.
When I delete this copied text, and re-save it, unicode disappears.

I tried to copy from Windows notepad and paste it to PL/SQL Developer, the problem can be reproduced, but not every time.

A file will be saved in Unicode format if it contains characters that are not part of the character set used by Windows. These characters would otherwise be lost. For example, if Windows uses a western character set and your file contains the following:
Then the saved file will be encoded in Unicode format.

I don't know why, but I noticed this behavior in the recent days (weeks), never seen this before.

We are using hungarian character set, the Windows is set to hungarian as default (not english language installed).
the codepage is Win 1250, which contains all the hungarian national characters.