that is exactly how it SHOULD work... from the kb...

Select the check box to use this field as the display value on the extended table.

For example, the Story [rm_story] table uses the short description as the display value in reference fields instead of the number, as defined in the Task [task] table.

https://docs.servicenow.com/bundle/kingston-platform-administration/page/administer/data-dictionary-tables/task/t_DefineADictionaryOverride.html

if that is not behaving correctly i suspect there is an issue and would open a hi ticket... the issue MIGHT be involved in the fact that the issue table extends the planned task table, which extends task... so they MAY not have the back ground process that does this looking at this correctly.