Pre-filled CI/CD variables feature is not working correctly for child pipeline

Summary

The default value for pre-filled CI/CD variable is not replaced by the new value specified during the pipeline run.

Steps to reproduce

1. Create a pipeline with predefined variable with some default value
2. Add child pipeline
3. Specify new (custom) value during manual pipeline run
4. Check and compare the variable values for both pipelines - parent and child

Example Project

https://gitlab.com/vvintoniak/pre-filled-variables-bug/-/pipelines/261220763

What is the current bug behavior?

For child pipeline: $ echo "VARIABLE value is $VARIABLE" VARIABLE value is DEFAULT

What is the expected correct behavior?

For child pipeline: $ echo "VARIABLE value is $VARIABLE" VARIABLE value is CUSTOM

Workaround

Redefine the variable during child pipeline triggering:

child-trigger:
  stage: trig
  variables:
    VARIABLE: $VARIABLE