From what I experienced yesterday it seems that the behavior for mix faders and midi faders is separated. The mix faders can be locked and the midi faders unlocked. The mix faders can be unlocked and the midi faders locked.
I did a search in the manual afterwards but didn’t find where it’s mentioned. So in case it’s not mentioned yet, I think it would be good to mention it in the manual at the fader lock subsection or the midi track section.
To make a long story short, my yesterday’s struggle was I used midi tracks for the first time and tried to control serato Dj with a fader’s CC but didn’t realized midi fader was locked for the reason above and also because in the submenu with all the midi settings the lock LED (F8) turns off.