Crack in mortar on internal leaf of exterior wall around pipe

Diagonal cracks usually appear in brickwork because of deflection , this could be due to settlement in foundations , soil movement , heavy vegetation , on the brickwork .

It's not a big problem you can rake joints out where cracks are wedge some natural slate in bed joints and repoint with sand and cement ok

Sounds like mortar shrinkage in the plaster as it is confined to a small area its nothing to worry about you can cut out the affected area (and rake out the mortar joints for a key ) and re-plaster pj

If you have no sign of movement on the external rendered wall then I wouldn’t worry to much about the internal movement, was the internal plaster finish cracked? question why have you removed the internal plaster for a stud partition move? If you hadn’t removed the base coat you wouldn’t know it was cracked!! Internal masonry of your property of that circa do suffer from internal leaf problems as the Newton block or brick strength are less the the external leaf strength and is very common. But if you have signs of deflection on the externals then you need to appoint a structural engineer.... don’t take a builders opinions!