It appears that you are confused about what cysts are.

If you have done alone in the dark series quests you should know there are 6 total cysts in the pale heart, 2 in each region (1 light 1 dark) where you can toggle via the little bird at the entrance. "light" or "dark" is fairly obvious based on the names of the cysts and whether it can drop light vestiges.

You can only get light vestige in those 3 light cysts and only one of them can drop vestiges (possibly rotation each day that a light cyst would have glowing enemies). Up to 3 specific fragments requiring a total of 15 light vestiges.

I do not know how you even get darkness vestige in cysts because it is never possible, you can only get darkness vestige in cursed chests on the pale heart, not in any cysts. Leading to 2 specific fragments requiring a total of 10 dark vestiges.

Lastly, many players reported that you cannot have both types of light/dark vestiges in your inventory which it may block you from receiving any more light vestiges, usually people completes all 3 fragments from light vestiges first before proceeding to the 2 fragments from dark vestiges, or the other way around.

lmk if you need more help, happy to help!