Start a text document and write some text. Apply a character style (not direct formatting) to a word, e.g. `Emphasis`. Enable recording change tracking (menu Edit > Track changes). Apply a different character style to the same word, e.g. `Strong Emphasis`. Save the document. Examine the part content.xml. Find the element <text:format-change>, child of element <text:changed-region> in element <text:tracked-changes>. The element <text:format-change> has an attribute loext:style-name, whose value points to an automatic style, likely `T1`. The purpose of the attribute loext:style-name is to backup the style, that was applied to the portion of text before its format was changed. Thus the expected value is the common style `Emphasis`. LibreOffice corrects the faulty value on opening the document, so that you get indeed `Emphasis` at the text portion when you reject the change. But the file format should correctly reflect the situation. And that was, that the portion has no direct formatting but a common style applied. Creating an automatic style in this situation is not needed at all. If you delete the automatic style and set `Emphasis` as value to the loext:style-name, then LibreOffice opens the file without problems and you can reject or accept the tracked change.
Interesting, thanks for the report. I can reproduce this. Note that sw/qa/filter/xml/data/format-char-style-change.odt is a very similar case, switching from Strong (~bold) to Quote (~italic) and that doesn't have this behavior. So the intent is already what you describe (no automatic style if possible), but you found a case where this is not happening.
I have noticed the problem, when I made changes to the default styles and when I use an own style.
I see, I'll take a look.
Miklos Vajna committed a patch related to this issue. It has been pushed to "master": https://git.libreoffice.org/core/commit/0c7eaa023a840a6f0967fe119463a936241a8130 tdf#168325 sw format redline, char style: improve recording It will be available in 26.2.0. The patch should be included in the daily builds available at https://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More information about daily builds can be found at: https://wiki.documentfoundation.org/Testing_Daily_Builds Affected users are encouraged to test the fix and report feedback.
Miklos Vajna committed a patch related to this issue. It has been pushed to "libreoffice-25-8": https://git.libreoffice.org/core/commit/1f6b2a4d6db61f0ae0f00774b3104cf679c3db4d tdf#168325 sw format redline, char style: improve recording It will be available in 25.8.3. The patch should be included in the daily builds available at https://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More information about daily builds can be found at: https://wiki.documentfoundation.org/Testing_Daily_Builds Affected users are encouraged to test the fix and report feedback.