BabaJ

It's because at frame 53 and afterwards, you have points arcing downwards. You've defined your point order as the distance from location 0,-1, 0 - so those points are now nearer the other ones which are higher in y value.

You will have to devise an equation or some other method for sorting your point numbers - depending on what the entire range of your geometry will be doing.

Here you go.
Just change values or target different attributes to suit your needs.

papsphilip
but I get the same result in a detail wrangle though

Are you sure?
Even in detail mode and if you use attributes, you can still trip yourself up; that's why I suggest you hold values to local defined variables, and do all the work on them first before setting the final attribute settings that should be separate(generally)and at the end of your code window in detail mode.

Otherwise, yes there may just be something amiss in the logic.
Again I haven't taken the time to look more closely on what you are doing but it does seem your code may just be a bit overly complicated.(no criticsim intended).

Perhaps if you attempt to redo your process in a more simplified way and post that - you might get better feedback.
You also might come up with something yourself that works better, or find your 'error', if that's the case.