A sort of magical Bio-Accumulation
We see this accumulation via consumption in the natural world. For every bit of fish a bigger fish eats, it also gains any pollutants that it's food had within it. A big fish can eat hundreds and thousands of smaller fish over their life span, only to then be eaten by larger sharks, who over time had eaten many large fish. What was but a speck in a tiny fish, is now thousands to millions of specks gathered into a single shark.
This is an actual danger for people who eat a lot of large fish, to make sure only to eat so much of the larger variety (sharks, sail fish, etc) as the amount of bio-accumulation of mercury within them can actually poison you as well as cause pregnant mothers and their babies to have issues.
Something similar may be seen when harvesting hearts to absorb all mana possible. Typcial witches starting down this path will likely attack and sacrifice lower magic beings before they start targeting powerful witches to steal their power. 227
Once a witch has eaten the heart of another, they essentially take in every bit of magical (or biological) pollutants of every being that witch had previously consumed.
Be it "dead-mana" or a bi-product that slowly builds from the use of mana "expended-mana" that never concentrates enough in a single user to cause a problem, but if you were to accumulate 100 times as much as a normal person could ever create themselves, now you've got too much dead/spend-mana weighing you down or causing magical mishaps.
A slow poisoning of mana may also be ignored by those obsessed enough with becoming powerful, accepting the slow decay or whatever negative effects, thinking that the gain in power is worth the tradeoff... any number of ailments could be given at varying levels of Hearts eaten.
A villager heart? No worry. A Witch heart? Take it slower, give a few years between targets to let the bad juju filter out of you to prevent too much dead-mana getting built up.
Mana rejection
Sacrificing a normal person that doesn't use magic is easy, they mind not even know about magic at all.
Once a person starts using mana, their ability to wield this power is a little different between users. Some call forth intent from within, some simply will the magic to perform, while some will use various tools and processes to get do what they need.
This puts a sort of flavor or method of use to the mana. Someone's mana may by more volatile as they channel their emotions to elicit effects that naturally flow from them in times of need, which could overwhelm a more concentrated and focused application of mana that much be pulled out from within rather than channeled as it's already bursting out.
This sort of mana "flavoring" etc may make it too dangerous to eat the heart of another witch without knowing what sort of methods they used to call forth their mana.
A flood of more lethargic, studious mana dumped into a witch that wields mana in a rage, might alter their own mana, or this new mana may essentially be wasted or unintentionally used because the method used by the witch doesn't reach this new variety.
Their heart might even actively reject the mana, seeing it as a poison of sorts and expel it via any means necessary (possibly including explosive results) 227