On Monday, 4 December 2017 22:13:16 UTC, Peter Smithson wrote:
I have a long plank of wood I could use. But even if I show that the frame is bent, they'll say it's the bricks that the frame is mounted on which doesn't sound right.

No brickwork is completely flat.
Frames are easily bent out of true by the fixing screws.
When door frames are installed, Packing pieces are put under it to make up irregularities/prevent it from being distorted. Either bits of wood or, these days, plastic.

Cement/ render may be additionally squeezed under it afterwards.
This is why door fitters/carpenters are skilled.
It seems yours weren't. Not uncommon these days.
It's definitely crap workmanship.