I’m using LibreOffice Version: 6.3.5.2 (x64) on Windows 7. In this and previous upgrade I started having sporatic problems with Find & Replace in Writer. Example: Find Marianna - Replace with Mariana produced MariannaMariana.
I finally figured out that this only occurs when I am Tracking Changes. If I turn off Tracking, Find & Replace works just fine.
I offer this here so anyone who is having a similar problem can use this workaround. Or if you have a different way to fix the problem, you can tell me. Also, I believe this is a genuine glitch and should be addressed in the next version.