I came across this issue by accident, I was remapping my Accapella, Rythem, Bass and drum stems to Pause as mentioned in my other thread, I also pushed a few buttons while in Midi window just to remind me what the midi mappings was, I did push a few buttons including 2 deck and 4 deck view, that was fine, then all of a sudden I was able to load songs onto decks that was currently playing,
I checked the DJay Pro settings and “protect deck” was still checked, I unchecked it and re-checked it, it still wasn’t working, I even closed DJay pro and restarted my Mac, the problem was still there,
a bit confused, but I know this issue happened because I did something 5 mins ago, so I went through and tried to push buttons on my DDJ SX2 that I had pushed before in the same sequence, I tried to load songs onto a playing deck to see if the protect deck issue was fixed, but it wasn’t,
when I did come across my 2 mapped buttons (2 deck and 4 deck view) I pushed 2 deck button first then 4 deck view button, I toggled both buttons a few times,
Then I tried to load a track onto a playing deck and it worked, I got the pop up that says this deck is protected, so my issue was fixed,
I tested it out a few times to make sure protect deck was working and it did.
I closed off DJay pro straight away and was just happy that I fixed the issue. I didnt want to mess it up again 
I don’t know if its the sequence of how I’ve mapped my DDJ SX2, including 2 deck and 4 deck view, That, I can be the only one that can reproduce this issue 
but this post can be for someone that has the same issue, hopefully this will help them,
Slak_Jaw I can send you my mapping file if you want let me know,