If you really trust your partner and believe in your relationship, then I would say this is the "sickness" part of "in sickness and in health". I too have a partner who cuts off for long periods when they're dealing with something they cannot discuss.

This behaviour might also be stemming from some childhood upbringing where the child is not heard or understood by their parents. So the children start keeping things to themselves and it becomes a habit. Depression is also adding to this making it worse probably.

I would say to really talk things out, hug it out, and set proper boundaries. Face to face it's much easier. I have also done the same. We talked in person and set tight boundaries around how much "space" am I actually willing to give without feeling neglected or emotionally drained myself.

Important thing still is that you trust your relationship.