Had a call with some technical folks today - the Custom Fields definition did change, from a key-value pair (strings), to an array of Custom Field objects.
We were told that the change was not planned or intended, and that it would be reverted in the next day or so.
As we didn’t know what was going on, we adjusted our integration code to handle the change (via trial-and-error, we figured out what had changed and how to address it). We’ll have to rollback those changes as soon as the integration “breaks” again.
Fingers crossed.