Hi @SeanK1 ,
Do you want to add a custom data card in your Edit form, then bind it to a specific field in your SP List?
According to the issue that you mentioned, actually, it is an known limit with custom data card. When you add custom data card in your Edit form, it would not own Update and Default property.
Only these data cards generated from your data source would have the Update and Default property. If you want to bind the custom data card to a specific field in your SP list, you could consider enable the corresponding field data card within your Edit form, then made some changes to it.
Currently, custom data card could not be bind to a field in your data source through the Edit form.
If you would like this feature to be added in PowerApps, please submit an idea to PowerApps Ideas Forum:
https://powerusers.microsoft.com/t5/Power-Apps-Ideas/idb-p/PowerAppsIdeas
Best regards,