Grief is a peculiar thing in some cultures. In the United States, the majority (white, Protestant) culture struggles with grief. We expect grief to be contained in practice, time, and scope. When people spend "too long" in their grief, we pathologize them. The criteria proposed for Persistent Complex Bereavement are culturally bound, but we know that the majority culture already pathologizes any culture outside what it considers to be the norm. The parentheses are my comments about the criteria:
Intense and persistent yearning for the deceased (How are we defining this? Is it not typical/to be expected that we miss our loved ones?)
Frequent preoccupation with the deceased (What's defined as preoccupation? Is talking to the deceased daily through prayer a preoccupation? What about visiting the grave site? What about ofrendas?)
The wisest thing someone said to me about grief was that the thing that makes grief so hard to deal with (for the non-grieving) is the idea that grief must have a time limit. This is especially true when someone experiences miscarriage, stillbirth, or infant loss. People, often well-meaning, good people, often respond poorly to the news of the death of a child. When someone is grieving, it's okay to tell them that you don't know what to say, but that you are there for them. It's okay to feel uncomfortable, but if your feelings about how someone "should" be grieving gets in the way of you being a good friend/sibling/therapist/coworker, step out of the way and allow that person to connect with someone who is able to set aside their own preconceptions about rituals of death and mourning and allow people the space to have theirs--even if (maybe especially if) the relationship between the griever and the deceased was difficult, strained, or even estranged.
NOTE: I came across an article some time ago that proposes that "autistic grief is not like neurotypical grief." While I love that this person shared their experience of grieving, it's important to recall that old saying: If you've met one person with autism, you've met one person with autism. And, whether typical or atypical, as long as the mourning is not physically harmful to the individual or people around them, everyone needs to have their own path through mourning.
Grief is a symptom of relationships, not pathology. If humans weren't relational creatures, 1) we wouldn't have survived as long as we have; and 2) we would be immune from grief.