Double slurs and custom part combining

[Jefferson Felix]

Hello,

I'm a little confused by the use of partCombine and doubleSlurs, when
I customize the merge condition with \partCombine #'(x . y).
The SATB scores I'm writing have matching rules that are very common
in music hymnals in my country, which separate notes up to the second,
and from the third onwards the notes are joined (I've attached an
example).
The problem is the use of doubleSlurs, which apparently doesn't work
very well when using the pair of numbers that customize this function.

So the example below

\version "2.22.1"

global = {
\key f \major
\time 4/4
\set doubleSlurs = ##t
}

soprano = \relative c' {
\global
f1 a1 (g2) g
}

alto = \relative c' {
\global
f1 f1 (e2) e
}

\score {
\new ChoirStaff <<
\new Staff \with { printPartCombineTexts = ##f } <<
\new Voice <<
\partCombine #'(2 . 20) \soprano \alto
>>
>>
>>
}


it's not generating doubleSlurs, but when I remove the pair of numbers
after \partCombine, doubleSlurs works.

Am I doing something wrong?

-- 
---
Jefferson dos Santos Felix

hymn-example.png
Description: PNG image