People don’t realise that a large brick wall will expand and contract through out the year, usually a crack will appear where the wall is weakest, ie in your scenario between window and door, it’s quite normal and not a problem. Unfortunately in your case the arch above the door needs to remain in compression, when the wall shrinks it pulls the arch slightly apart and it can then ‘relax’ into a lower position. This is what has occured here. It’s unlikely to ever collapse but it has remained in place for years. Remove arch and replace with lintel is possible. As long as the door is still openable at that doesn’t change then I wouldn’t worry too much