Okay, that makes sense.
My next guess would be that you are missing/misconfiguring sonar.java.test.libraries, is that possible?
In case it is missing, you should see a message in the logs of the analysis:
WARN: Bytecode of dependencies was not provided for analysis of test files, you might end up with less precise results. Bytecode can be provided using sonar.java.test.libraries property.
When it is misconfigured, the message will not appear but the result will still be less precise (we are working to improve the logs as we speak). You have to make sure that all test dependencies are correctly provided.
For example, in my sample project reproducing your case, I added sonar.java.test.libraries=dependencies in the configuration, and made sure this folder contains junit-jupiter-api-5.7.0.jar.
By the way, you can also check that sonar.java.libraries is correctly configured aswell, since it is the equivalent for main files.