OSX: Not possible to assign the function-key (fn)

In the Minecraft-versions 1.12.2 and older it was possible to assign OSX´s function key as a control. (Not F1 - F12 but the one labeled "fn")

In the "options.txt"-file the fn-key had the id 196.

(I used it for the sprint-action.)

In Minecraft 1.13.2 and newer snapshots this is not possible anymore:

If I assign the key in the version 1.12.2 and then load the 1.13.2 it gets set to "Left Control". I guess this is just the default key for sprinting.

If I try to assign it in the version 1.13.2, pressing the fn-key just gets ignored.