Hi @Amy_Bozier 
and thank you for reaching out!
You are correct here. The reason why you are not seeing Subtasks in Workload Subtasks is because Subtasks do not currently inherit the project from their parent tasks. You can quickly add them to the Project and once you do it, they will be displayed in Workload. You can learn how to do it in this guide article.
Regarding your question about Custom Fields in Subtask, I’ll recommend you to have a look at this thread: Custom fields needed on subtasks. Feel free to add you vote and comment there.
I hope this helps! Please let me know if you have any follow-up question!