Hello,
there is some flawed logic in the JUCE code that handles patching the JACK ports of the JUCE app to the ports of the selected JACK “device”.
Basically, the code iterates over the set bits of the inputChannels/outputChannels BigInteger instances, but these get modified while iterating because JACK calls back to notify that the previous channel was correctly patched.
Here is a pull request that fixes the issue:
Cheers