I Have Kde-Neon 5.11.4 installed and when I open an .odt file it always open it with Okular instead of LO Writer. I change this behavior on Settings --> Applications --> File associations and in Dolphin --> Riht Click on an .odt file --> Properties with no luck. At first it seems that the behavior changes but at next reboot system opens odt files with Okular again. It only happens with .odt files, not ods or odp ones, and I think it is because when I installed Kde-Neon on my computer a week ago, I opened an odt file that I have, with some notes about what I use to do (configurations and install software) after a fresh install. I did prior to install any additional software of that comes installed by default and without LO Writer installed on my system Okular opens that file. That's why now Okular is the default app for opening that files and I can't change it because I think there is a bug and the system does not rembember the new file association.
Further information: Sometimes when I try to promote Libreoffice Writer at first place for opening odt files, the setting was not saved when I press "Apply". I have to try several times. What works instead is to send down Okular to the second place, but how I said before, this setting does not remains after reboots
After some days I have observed that this bug seems solved on my computer. I don't know if it is because of some update or what. Since three or four days, when I start my system and try to open an .odt file it opens with LO Writer as it should and this behavior remains the same for this last days. Perhaps this bug might be closed but I don't find where I can do that so I'm going to mark it as solved. If the bug reappears I'll reopen it.
Thanks for the update!