New thermostatic mixer shower installation - only extremely cold or extremely hot water coming from it?

Had a thermostatic mixer shower installed in recent bathroom renovation, but the water is either extremely cold or extremely hot. The left inlet pipe is cold to touch, the right inlet pipe is hot to touch. Likewise the bath and basin taps are labeled H (left) & C (right) but water coming from them is cold on the left tap and hot on the right tap.

I mentioned this to the plumber and he said he just put the flexi pipes on as they were coming in. Surely these could be swapped around and bath and basin taps are sorted? What can be done about the thermostatic mixer shower though? As the plumbing for that is behind a thick sheet of adhesived PVC and tile. Ideally I don't want to be ripping out the PVC and tiles beneath tbh as it's all brand spanking new.

Previous owners have said they've always had water in the wrong way i.e. C left & H right. They also had an electric shower so no issue with water temperature.

The problem is with this bathroom only and the rest of the taps in the house are the right way around.

Instead of swapping around the flexis, can this not be a straight swap "at source" - if yes, where would I find the source?

Anything else that can be checked or done? I can live with the taps being on the wrong way around but not the shower given I've spent thousands on the reno.

I asked the plumber to come back and fix this but obviously he's being reluctant after being paid in full. The work was done a couple months back and unfortunately I was hospitalised so only got to check this after we moved in a few days ago.