This is a known issue with LBG (Lloyds Baking Group). They have an internal process which checks each transaction for fraud. While a transaction may be "BOOKED" it may still be subject to its fraud check this is why some transaction ids are prone to changing as the transaction moves from an unchecked database into the checked database with a different id.
LBG will be making some changes to infrastructure scheduled for Q3 2020, which will bring this process all into one central system and the transaction ids should become immutable.
20-01-2020 - Reported discrepancy in transactionId to LBG via OBSD
19-02-2020 - LBG confirmed as known issue and reported Q3 2020 fix.