In one of my AND statements i test whether or not a row has a SKU, this particular sku has an open parentesis as such: 

AND(@Lookup,H:H;="Savings (fixed term & purposed based")

Even though the function should treat everything in quotation marks as a string, it misinterprets it and forces a closing parentesis onto the end of the function. So instead of correctly being like this: 

sum(map(cumulative(repeat("1",count(select(@Lookup,D:D;,and((@Lookup,H:H;="Savings (fixed term & purposed based"),(@Lookup,E:E;="DK")))))),"temp",sum(select(@I:I;,and((@G:G;=slice(array("1",select(@Lookup,D:D;,and((@Lookup,H:H;="Savings (fixed term & purposed based"),(@Lookup,E:E;="DK")))),$temp,($temp+"1"))),date_in(date(@A:A;,"dd-MM-yyyy"),"4","-1"),(@H:H;=blank()))))))

It corrects it to: 

sum(map(cumulative(repeat("1",count(select(@Lookup,D:D;,and((@Lookup,H:H;="Savings (fixed term & purposed based"),(@Lookup,E:E;="DK")))))),"temp",sum(select(@I:I;,and((@G:G;=slice(array("1",select(@Lookup,D:D;,and((@Lookup,H:H;="Savings (fixed term & purposed based"),(@Lookup,E:E;="DK")))),$temp,($temp+"1"))),date_in(date(@A:A;,"dd-MM-yyyy"),"4","-1"),(@H:H;=blank()))))))))

To account for the open parenteses in the string.