Consider a span of bold or italic text within a larger run of normal text.
If you select the start of the italic/bold text and type over it to change the word(s), instead of the replacement text staying bod/italic, it is given normal style.
It appears that LO is adopting the text style from the text to the left of the start of the run of text being replaced, instead of the style of the text it is replacing.
(If the text selected is not at the start of the run of bold/italic text, the replacement text is correctly given the style of the text it replaces.)
*** This bug has been marked as a duplicate of bug 79717 ***