How do you manage allergen risk when a supplier changes their ingredients without telling you?

[Marzena STC & QTC]

Hi all, just wanted to raise something that I think could catch a lot of businesses out. Suppliers sometimes quietly change their formulations, processing aids, or allergen warnings and the customer only finds out when a new spec sheet arrives, or sometimes not at all.
Would love to hear from people who've dealt with this, what processes or agreements do you have in place to make sure suppliers notify you of any changes? And how do you make sure your own labels and risk assessments stay up to date when that happens?
Really interested in what's working practically for people, rather than just what the standard says on paper