I asked the Shopware AI copilot, and that's the response:

The template hierarchy for apps and plugins is as follows:


- Shopware Core templates
- Theme templates
- Plugin templates
- App templates

This means that if a template is present in both a plugin and an app, the app’s template will take precedence.
If a template is present in both a theme and a plugin, the plugin’s template will take precedence.
If a template is present in all four locations, the app’s template will take precedence.

Could it be that some plugin and/or your child theme "destroys" this hierarchy by not using the {{ parent() }} call in a block?