Config not reloaded

### Task List

- [x] Steps to reproduce provided
- [x] Full description of the issue provided (see below)

### Steps to Reproduce
1. Open any project with an inline plugin
2. Change any config property in project's `application.groovy`
3. Debug `onConfigChange` in plugin file

### Expected Behaviour
`event.source` should contain the config with the changed value

### Actual Behaviour
It doesn't reload config

### Environment Information

- **Operating System**: Windows 10
- **Grails Version:** 3.3.6
- **JDK Version:** 1.8.0_171

Reproduce the issue with an inline plugin by changing a property in the project's application.groovy and debugging the plugin's onConfigChange callback. Trace why event.source does not contain the changed value. Done means the callback receives the reloaded configuration after the property changes on Grails 3.3.6.

Written by the indexing model from the issue text.