When the arch gives up
The brick arch over this doorway had failed — and once an arch loses its shape, the brickwork above it has nothing holding it up. You can see the cracking stepping through the courses in the "before" photo.
The fix isn't filler. The load came off, a new lintel went in to carry the wall properly, two sandstone lintels were installed, and the brickwork above was rebuilt to match what was there.
Done right, a repair like this shouldn't announce itself. It should just look like the house.