Costs imported even though PA Period Closed - how?

Summary:

Costs imported even though PA Period Closed - how?

Content (required):

The Mar-23 Projects period was closed on 3rd July on PROD.

On 4th and 5th July, someone went into Projects -> Costs -> Unprocessed Transactions and clicked into the exceptions for a custom Transaction Source.

The exceptions were due to the following - example here for one expenditure item:

From there, click Import Costs:

The expenditure item was created successfully, and we can drill into the expenditure item via Manage Project Costs and view the Costing info for it, confirming that the period is against Mar-23:

What I can't work out is how it was possible to import the costs even though the PA Period was closed on 3rd July.