The newly generated rt/build/compile.args file uses --upgrade-module-path to point to the recently built local javafx.* modules. This does not correctly pick up any modified or added classes, presumably because the javafx.* modules in the JDK are not upgradeable.
Instead we probably need to use --patch-module as is done in the run.args file. If I take run.args and strip out the -Djava.library.path (which I thought was the intention for compile.args) then it works correctly for a simple test case.
The testcompile.args will likely need the same change.