Greetings,
after some testing with a fellow healer I came to the conclussion that stacking critical defense with the Impenetrable trait on armor pieces reduces the healing taken from critical heals. If you want to test this for yourself, keep in mind that changing the number of heavy armor pieces on your character can impact the testing, depending on how many points you have spent on the passive ability Rapid Mending.
I haven't tested the champion trait "Resistant" yet, which might also affect the amount of critical heals.