I recently acquired SPARC T4 hardware which allows me to run
Solaris-11.4 CBE and this issue with net/tigervnc appears to be fixed
by the recent Makefile change:
# CMake build appends 'tigervnc', so this is not PKG_SYSCONFDIR
CMAKE_CONFIGURE_ARGS+= -DCMAKE_INSTALL_SYSCONFDIR=${PKG_SYSCONFBASE}
So this bug can probably be closed.