i have checked the template but we are not doing nothing on our side to change your formula, the output for me looks like this:
=LOOKUP(2,1/(INDIRECT(CONCATENATE("'",INDIRECT(CONCATENATE("Z",ROW(),"S1"),FALSE),"'!D:D"))<>""),INDIRECT(CONCATENATE("'",INDIRECT(CONCATENATE("Z",ROW(),"S1"),FALSE),"'!D:D")))
and in MS excel (which it displays according to my local, in Spanish)
=BUSCAR(2;1/(INDIRECTO(CONCATENAR("'";INDIRECTO(CONCATENAR("Z";FILA();"S1");FALSO);"'!D:D"))<>"");INDIRECTO(CONCATENAR("'";INDIRECTO(CONCATENAR("Z";FILA();"S1");FALSO);"'!D:D")))
i don't see the @
being added
however what i've noticed is that internally the formulas are being html escaped, this is likely caused by your custom formula
formula helper, i recommend you to change the formula helper call to use triple {{{
, update your calls to {{{formula ....}}}
maybe this fix the way your MS Excel parse the formula