Treating You Like You're Invisible Or Incompetent

The skewed and/or absent feedback from narcissistic friends, family members, and partners can cause serious anxiety and depression, especially when one cares deeply for the other person.
Skewed feedback can come in the form of criticism and insults; for example a person who completes a painting, and the "feedback" they receive from their so-called friend is critical and insulting.

It can come in the form of pretending, such as pretending a person is not capable of, experienced in, or knowledgeable about certain things that they actually are. For example when a family member is an obviously experienced animal caregiver, but other family members pay a kennel instead of asking them to watch their pets while they go on vacation. (Completely different from "not wanting to burden someone", especially since they often make other demands on the person, and rarely give any indication of "respecting their space" any other time.)

It can come in the form of passive-aggressive advice-giving, which is meant to purposely give the message "You don't know what you're doing or talking about, you're not that bright, you're just a novice and a beginner", such as giving basic cooking advice to an experienced cook, or giving basic photography instruction to an experienced photographer, or explaining how to change automotive oil and why it's important to a person who is in the process of changing their oil. Explaining what frets are to a person who plays guitar. Explaining 'what it's like' to be a writer to a person who is a writer. Giving detailed advice and instruction about repairing a hole in a wall to a professional contractor.

It can come in the form of "giving help", which is often used as a domination weapon, implying that the "helper" is far and away more capable, more stable, and/or physically stronger than the other person, and that the other person CAN'T do the task without assistance.

Absent feedback is when they simply act as if the person is invisible, or that nothing the person does is visible. 
They may keep talking and behaving normally, as if nothing is happening when a person is in obvious distress.  
They may behave as if nothing at all happened, directly after the person just suffered a tragedy, a serious violation against them, an injury, a serious illness, or some kind of major difficulty. They may completely ignore a person's presence.
They may totally refrain from acknowledging a person's accomplishments and milestones.
They may ignore invitations from this person.
They will often not invite this person to celebrations or gatherings, as if the person is not a "real" member of their group/family, or a "real" person.
They often behave the same way toward the person's children.
They may simply ignore communications from the other person.
They will usually completely deny and mentally "delete" their own behavior toward the person, and use the person's reactions as "justification" for continued disrespectful treatment, such as:
Because they have left the person out of normal family or friend events and celebrations, the person no longer tries to keep in contact with them, and has stopped trying. So they will say that the "reason" they don't invite the person to anything is because the person doesn't seem to "care about them", because they don't keep in contact or show up to any events. They simply delete the reality that they have been treating the person like a non-member of the family and THAT'S why the person stopped trying to keep contact with them. As far as they're concerned, they have accomplished their goal of ostracizing the person they had been targeting all along.

This is all literally an attempt to either make the person "smaller" in their life, or themselves "bigger"/"more important" than the other person. It's often motivated by either envy, fear (either of the person, or fear of someone who doesn't like the person), or a serious supremacy complex.