Version: 6.2.8.2 I am using Mac OS X Mojave version 10.14.6 but this has been happening on prior versions. I currently must use the Find/Replace to search dialog.
This is no repro for me on LO6342. Cmd-F should bring up the Find bar at the bottom of the LO document window by default. This is built in functionality and works as designed. The fact that LO on macOS does not use native dialogs is a different issue.