Complete module splits not enabling modules

That sounds like a critical bug!
After all this is the fundamental feature of this module. Given that this used to work and that Config Split hasn't changed in a while, I am tempted to think that there is a related change in Drupal core. I have run into some config validation issues with another module I am working on, but I didn't yet have time to get to the bottom of it. I have some hunches of what goes wrong though so I will look into it soon.

Hi, we encountered a similar problem using Drupal 10.x (10.4.x and 10.5.x) with Config Split 2.0.2. During drush cim, modules defined in the local split wouldn't be imported.

We use a split parent :

/* config/parent/config_split.config_split.parent.yml */
...
weight: -100
stackable: false
no_patching: false
storage: folder
folder: ../config/parent
module:
  config_split: 0
...

In this split we define all our splits (eg local)

/* config/parent/config_split.config_split.local.yml */
...
weight: 0
stackable: false
no_patching: false
storage: folder
folder: ../config/local
module:
  dblog: 0
...

The issue was resolved when we changed the parent split’s weight from –100 to 0.

We also set up a fresh Drupal 11 instance but could not reproduce the issue described by debra-v there.

Could you please share precise reproduction steps for the issue?

I was not able to reproduce this bug. could you please elaborate on how to reproduce it?

I just upgraded from 1.x to 2.x and I can no longer completely split. I'm not patching or stacking. Perhaps I'm not understanding how it works in 2.x.

For example, I want the Email Reroute module in my dev environment, but not in production. What is the right way to achieve this now?

I have multiple splits to handle a multisite code base, one for each site, and multiple server environments (dev, test, live. At any time, 2 splits are applied in addition to the shared configuration in config/sync. All splits have weight 0, no patching, no stacking.

site1:
module:
unpublished_nodes_redirect: 0
tvi: 0

site2:
module:
unpublished_nodes_redirect: 0
tvi: 0

site3:
module:
unpublished_nodes_redirect: 0
tvi: 0

ddev:
modules:
devel: 0
field_ui: 0
update: 0
views_ui: 0

pantheon-dev:
modules:
devel: 0
field_ui: 0
update: 0
views_ui: 0

pantheon:
modules:
pantheon_advanced_page_cache: 0

The error appears at drush cim when the config files for the modules specified in the splits are attempted for import.

It seems that when config_split module configs are broken out into their own split (this used to work?), the modules for other splits are not enabled as expected despite this split being active and enabled for all conditions.

...
langcode: en
status: true
dependencies: {  }
id: config_split
label: 'Config Split'
description: 'Config Split configs'
weight: 0
stackable: false
no_patching: true
storage: folder
folder: ./profiles/custom/my_profile/config/config_split
module:
  config_split: 0
theme: {  }
complete_list: {  }
partial_list: {  }

Eliminating this split, re-adding config_split to core.extension and returning all config_split configs to the general sync directory resolved the problem with the modules not being enabled for other splits.