As is usual in such a circumstance, you applied some direct formatting to your “unchangeable” paragraph.
To fix the problem, select the whole content of your problematic paragraph (your “frozen” Heading 1 paragraph) and Format>Clear Direct Formatting or Cntrl+M.
The reason lies in the precedence of styles:
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at the deepest level, attributes are set as per Tools>Options, various parameters, among which Language settings>Languages and LibreOffice Writer>Standard Fonts
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next paragraph styles organised hierarchically (they all depend ultimately on Standard Style)
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next character styles
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finally direct formatting, i.e. attributes you set manually with buttons and menu items.
Each layer overrides those beneath. Direct formatting being the top layer is overridden by none and must be explicitly cleared. Otherwise it hides the corresponding attributes set by styles.
If you want to be immune to such annoyance, avoid direct formatting. It is the root of all evil. This means you must design your own set of consistent styles (i.e. customising at least the built-in styles to fit your needs) in paragraph and character categories at minimum.
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