Cracks during box frame installation

[Kiaara]

We live in a 1930’s semi detached and currently extending the house which includes installing a box frame to open the kitchen. There are some cracks that have appeared in the first floor hallway when the installation was taking place. The builder doesn’t seem bothered but should we be worried? Pictures below

Pictures

It's hard to tell from a picture, being honest you should expect some cracks even with the best will in the world as the beams are designed to flex under the weight of all the bricks etc and that flex causes cracking. The amount is specified by the engineer to be small enough to not cause any issue other than cosmetic.
The main thing is as long as the movement was a one off and not progressive, the builder will patch it all up as normal and maybe adjust the doors a little.
If you really are concerned you can get the building control inspector from the council to have a look, they should already have the drawings and calculations and likely know a bit about the progress. They will be able to tell if it's a major issue and since you've already paid they would likely come out on a normal inspection the next day. But don't hold me to that.
Hope that's helpful

Thank you @johnd2, really helps

Some minor movement and settlement is to be expected when doing this kind of work. Listen to the builder, nothing to worry about. Just means a bit of bit of plaster repair required once work is finished.