If the blue PoS received can only be registered in the UDB at the start of the next mass balance period, how can we supply the PoS within 30 calendar days, as required?

The Proof of Sustainability (PoS) is created at the point of creating a transaction. PoS details should be updated before the end of the mass balance period or before submitting the transaction. In either case, this also fulfills the needs for 30 calendar days specified by the scheme.