Cannot simulate modified or removed transactions using a custom sandbox user.
I can create a custom user with transactions and load the data in to my app, but when I edit the transactions of the custom sandbox user in the plaid developer portal and call transactions/sync for the item with the stored access token, the "modified" and "removed" response arrays are empty.
Custom users can't be used to simulate transaction updates in that way. The closest you can do is to use the date_transacted field, transactions in Sandbox will move from pending to posted once their date_transacted field has been reached. But you can't update an existing custom user by modifying the custom user json.
The best way to simulate realistic transaction updates in Sandbox is via the user_transactions_dynamic test user. When using that user, every time you call /transactions/refresh, realistic transaction updates will be simulated.
For more details, see the docs: https://plaid.com/docs/transactions/transactions-data/#testing-pending-and-posted-transactions