trigger:forward doesn't work in scheduled child pipeline

Related:

• #213729 (closed)
• #357499 (closed)

In a scheduled pipeline, trigger:forward doesn't seem to work with child pipeline.

MRE: https://gitlab.com/garyng/trigger-forward-grandchild-mre/-/tree/mre/scheduled

Scheduled job setup:

Pipeline run: https://gitlab.com/garyng/trigger-forward-grandchild-mre/-/pipelines/507910913

Expected result:

TEST_VAR is test_var
TEST_VAR_UI is test_var_ui

Actual result:

TEST_VAR is test_var
TEST_VAR_UI is <default>