Can't see from the vid if it's the brass bracket that's fixed to the wall that's also moving along with the wet wall or if it's just the brass riser, that's fitted into that bracket on the wall (with the lever that fixes the adjustable height riser) hasn't had the allan key grub screw tightened properly.
Either or .... no - the vertical riser (edit) shouldn't move/twist to that degree.
OK, if it's the wet wall that's flexing then it may not have been fixed to the subsurface properly at that point. If the shower was done as part of the same job as the wet wall, then they should have made sure between them, that wherever the fixing points of the shower was going to be that they made sure there was plenty of adhesive for the wet wall at those points and/or if it's plasterboard behind it that there would be a firm fixing for it all at those points, so the bracket could be screwed through nice and tight.
The question for you now is - does the wet wall come off and get fitted properly or what other option can they suggest because that amount of movement could ultimately cause problems over time if the shower riser/handset is used/moved frequently and it flexes to that degree.
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