In which markings.cfg file did you write your code?
There is one in the main FeralHeart folder, and one in FeralHeart>Media>Textures - you should only put your marking code in the markings.cfg that's in the main folder.
I've recently noticed that there is a certain limit to how many markings you can have as on the end of my mass markings, some of them show up as default markings so when I've added a custom marking to the end of the list, it does the same. So, putting my custom marking near the top of the list in my markings.cfg meant I could then see my marking. Hope this helps! ^^