Not a solution but some tips from my experience on this - farming the start of level 3 with the three prisoners is by far the quickest way to obtain this. (every 20 brains = 1%)
I strongly recommend doing this when you have 2-3 hours spare and knocking this out in one sitting. (restarting the level instead of loading checkpoint is recommending to limit crashes) credit : JeremyHBPhD
I was having problems with the tracker locking up and not progressing - I believe and it's only my opinion that if you back out to the main menu or shut the console down, it does not save all of your progress.
For example, if your stuck @ 75% like I was, it is possible that you will have to eat another 1504 brains for it to start registering on the tracker again, at least that is what happened to me.
Good luck.
Edit 12/04/21 : It's been suggested that by completing the level and backing out at the start of the next level saves your progress. Maybe worth doing this if your game crashes frequently. - Thanks xxxIsaiahxxx for this additional tip.