However, looking at the code in CuraEngine, it appears that in surface mode the lines in each layer can either be closed loops (aka polygons) or they can just be lines that don't join up. The polygons will be printed taking into account the z-seam position but it appears that the lines ignore the z-seam position. Perhaps that could be changed, although it's not immediately obvious what the right strategy would be.
Here on 4.5 z-seam alignment seems to work as expected (tested with user specified). Maybe you're using 4.6 which suffers from a regression concerning z-seam alignment: https://github.com/Ultimaker/Cura/issues/7557
It's supposed to be fixed already in 4.6.1
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