Xcode 10, by default, doesn’t properly rebuild the JUCE sources. This is probably due to it being confused by
.cpp files being
#included. This causes bugs and headaches (“aaahh why didn’t it show my debug print!?!”).
Things however still work fine if enabling the “Legacy Build System”.
I suggest that the Projucer should save the project with this setting, and this commit (from SR’s juce5 branch) makes it do that. If and when the problem is fixed in the future via Apple changing their build system or JUCE adapting itself to it (I suspect the legacy system will be dropped at some point), this change should be removed.