From Scene Builder's code, defining Help menu content:

toolHelpMenuItem.setAccelerator(new KeyCodeCombination(KeyCode.F1));

Since we moved the menu bar as a native one the accelerator no longer reads when opening the Help menu.
All other accelerators show up correctly, typically Meta + <[A-Z]>.
Looks like it could be specific to accelerator using Function key ...
Surely this isn't related to the fact it's Help menu because when I use KeyCode.F1 to replace say the Edit > Copy accelerator it doesn't read as well.