From The Document Foundation Wiki
When I enter text in lower-case, LibreOffice changes the first letter to a capital.
This behavior seeks to correct a sentence that you have forgotten to begin with a capital letter.
To remove this automatic correction:
- select ▸ ▸ ▸ ;
- near the top of the list, uncheck Capitalize first letter of every sentence; and
- click OK to apply the changes.
To learn more about automatics, please, read this FAQ item 025 – These automatics annoy me!