My heart just broke...
I also tested with a temporary preset call "Test" and created a js button with a js action.
Back it up and restore the test preset
And you can see the code in js action is emptied.
I am feeling really depressed because I didn't save the code elsewhere. (I opened the backup with TextEdit, no code found)
For those who have any js actions in the presets, DO NOT RESTORE ANY BACKUPS currently.