Sorting loops by a subfield in a nested group

Hi,

I would like to sort loop items by the value of a number field in a nested group (2 levels down).
I can sort by top-level fields without problems but not by nested subfields.

Something like this obviously doesn't work:

{% set args = 
    { 
    post_type: 'my-cpt', 
    posts_per_page: -1, 
    meta_key: 'my_group.my_group_2.my_subfield',
    orderby: 'meta_value', 
    order: 'ASC', 
    } 
%}
{% set posts =  mb.get_posts(args) %}

Using only 'my_subfield' doesn't work either.

I also tried to sort using Twig's "|sort":

{% set sortorder = '' %}
{% for post in posts|sort((a, b) => a.sortorder <=> b.sortorder) %}    
{% set sortorder = post.my_group.my_group2.my_subfield %}

But that won't work, either.
How can I sort by a nested subfield?

Greetings,
Tom