Why do some vendor prices appear red on Purchase Orders?

On Fourth, the 'Vendor Prices' for some products listed on purchase orders appear red.

This can be seen when opening a PO in 'Receiving Center' or 'Receiving Corrections'.

We thought this meant that a product's price has changed sometime after the order was placed. But I've spot-checked a few orders and found that isn't the case.

e.g.
We have product 5477, which is priced at £11.26 per CASE on the PO 910. This order was placed in Dec 2024 and received in shortly after.

On the PO, the price for product 5477 is also highlighted red, suggesting the price has changed sometime after the order was placed.

However, when I look at the bid history, I can see that the last bid for it went live back in 2023, long before PO 910 was placed. No other bids for this product have been added since.

Please could someone clarify why some vendor prices are highlighted red on POs. If this looks like a bug on our system, please could you let me know and I'll log it.

Any help is much appreciated. Thankyou