I have Catalina. I installed JDK 14.0.2 and under preferences and advanced in LibreOffice it shows it as installed and it’s checked showing that LibreOffice is using it. However, while I can open up Base, when I try to use the wizard, I get a message that this version of Java is not compatible. I’ve tried reinstalling Java, I’ve tried uninstalling Java, but have been unable to find either terminal or GUI commands to remove it. Most recently I tried, sudo rm -rf /Library/Java/JavaVirtualMachines/openjdk-14.0.2.jdk but it doesn’t seem to remove Java as when I open LibreOffice it still shows 14.0.2 as selected under Advanced.
Have you tried looking at the relevant folder contents (under the FINDER Go MENU) /Library/Java/JavaVirtualMachines ? You should see a separate folder for each jdk you have installed.