This isn’t my personal area of expertise, however, I have found the API reference documents useful in the past. You can check out what they have to say about Subitems and the API queries here, I hope that is as helpful for you as it is for me!
I think this is a regression. They didn’t implement the display_value for this to return it.
@rachelatmonday this looks to be a regression, there is no ... on SubtasksValue { display_value} to return a text value of the names of all subitems. I suspect this is an easy fix, since the logic is very close to that of BoardRelationValue.
Then you’ll have the subitems array, which you should be able to map the name field to it a new array, then join with commas to get the string. Annoying, but possible assuming you’re working with a language that will permit this kind of work.