Current State:

Any parent tracker that wants to set trigger on to a child's selectbox field,
will get a long column of options starting with actual child selectbox values AND followed by ~30 'undefined' ones. See attachment

Desired state:
you can deside. This is a cosmetic defect not affecting usability much.

SIDENOTE: there is a problem with Workflow > Triggers when importing parent trackers.
The selection fields just after words 'of type', 'set to' and 'will be set to' will become "<>" instead of original Child tracker fields like "User Story", "Closed" and "Closed", which is obviously unusable for the person who imported the parent tracker or a project template.

Hence, a warning on export attempt would be nice to see, i.e. warning showing something like "This tracker Workflow Triggers will need to be manually reset after import into another project".

OR, just remove the Workflow Triggers from attempts to XML export, and display a warning: "This Tracker's Workflow Triggers have been excluded from export".