That does sound a bit strange. My understanding of Adjustment Disorder is that is used for something temporary. Six years seems like a long time for symptoms for that label.

I know in the U.S. sometimes adjustment disorder is put on insurance paperwork sort of as a placeholder, as in 1. there is no other disorder but something needs to be on there for insurance to pay out, or 2. it's put on there while they figure out a more permanent diagnosis/see how the client progresses. Honestly, it may just be a "paperwork" thing.

Are you still seeing a doctor or therapist? Could you ask them to explain?

I know it is easy to get wrapped up in labels and that sometimes it is easier to cope with symptoms if you have an explanation. So for that reason I would encourage you to ask questions. At the same time, please don't feel like this somehow invalidates what you've been going through.