Note that this updated Java distribution: On the three systems I tested thus far, it appeared to fix the issue on one of the systems, but not fix for the other two. Unfortunately, the currently employed tweaks do not appear to solve the issue on all If you wish to test it, you can download the most recent Fiji for macOS and let us know here whether it works for you. The macOS version of Fiji is now distributed with AdoptOpenJDK 1.8.0_202 with tweaks intended to prevent macOS from complaining.
I am working on an update to the macOS version of Fiji to avoid this issue. Yes, it is an issue with how the ImageJ Launcher works, together with how macOS handles attempts to link to the libjvm.dylib shared library.
I think the default launcher has a problem and somewhere is still calling for Java 6
