How to reproduce:
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1. Create new Impress Presentation.
2. Go to the "Edit Master..." view.
3. Observe that in the bottom right of the master, there is exactly one "<number>".
4. Save the document as "test.fodp". The file save dialog will automatically change the file format to "Flat XML ODF Presentation (.fodp)".
5. Close the file.
6. Reopen the file.
7. Go to the "Edit Master..." view.

Expected Result:
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Observe that in the bottom right of the master, there is exactly one "<number>".

Actual Result:
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Observe that in the bottom right of the master, there is more than one "<number>". There is also more than one "<footer>" and more than one "<date/time>". If you observe closer, there is also more than one text box (the main text of the page). You can verify this by dragging the text box to some other position. Then it will be apparent that there is actually another text box.

Analysis:
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It seems that LibreOffice Impress tries to recreate what is already there without looking. The problem does not seem to appear for the .odp format, which hints to seemingly similar, but different code paths.

Impact:
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This makes using the .fodp file format unusable, which is a problem, because this is the only LibreOffice Impress file format which is suitable for revision control systems such as git or subversion.