Table of Contents Indexes change "levels" by themselves

SOLVED: Thanks to mikekaganski, the root of the problem was tracked down to using external file format, in this cas docx. When converted to odt, the issue got fixed.

Hello everybody. I am writing a thesis on Writer, and I have created a table of contents. The procedure that I follow is: Writing a new paragraph, select it’s title then select “insert” → “table of contents and index” → “index”. I select “table of contents”, then the level, and then ok. I then right click the Table, I select update, and all seems OK.
But, when I close and reopen the document, all the indexes have upped themselves by one level. I don’t know why this happens, any help will be welcome. I have already googled for an answer but did not find any similar incidents. In order for you to understand better, I will upload a screenshot

Is it using some external file format, e.g. docx?

Also what happens if you update the index?

Yes, I am writing in .docx because my professor uses MS Office. Do you believe that this may be the issue ?
When you say “what happens when I update the index” do you mean if I make changes on the indexes and then update the TOC? The changes show… If I update after a reoppening of the doc, the indexes move up by one level

Yes it might be the problem. I would advise you to file a bug report.

Thank you very much! I will try on the native file format, and select your solution if it works