The payment is stuck in Submitted status, why?

It is not uncommon for payments to remain in the "Submitted" status for a while before they are moved to "Executed" by the bank.

This delay can occur due to various reasons, primarily dependent on the bank. However, the most common reason is the RFI checks. RFI stands for "Request For Information," which is a process used by correspondent banks or other payment service providers (such as banking partners), where they may require more information on specific transactions.
The reason for this is to ensure that TrueLayer are compliant with regulatory responsibilities.

The time it takes for the checks to be completed and the payment status to change to "Executed" can vary depending on the specific scheme and country involved. In some cases, it may take up to four business days for the status to change to "Executed" and reach a terminal status.

However, if the transaction is being processed through the Faster Payment Scheme (FPS) in the UK, the payment can take until the end of the following working day to be marked as "Executed."

For reference, the scheme declares on this Faster Payments page:

As a minimum, all financial institutions in the UK must abide by the Payment Services Directive (PSD) -  which states that any payments made by mobile, internet, or phone banking (including standing orders) must arrive by the end of the following business day at the latest.