The organisational half, which is the actual root cause and the only permanent fix: the team that owns the source did not know you depended on that column's meaning.
From their side, adding a new status value is a completely ordinary product change. It broke you because your dependency was invisible to them.
What helps:
Tell them the dependency exists. Simply knowing that finance reporting reads that field changes how people behave. Most upstream teams are happy to give notice and cannot give notice about something they do not know about.
Ask to be on the release notes or the channel where schema changes get discussed. Low effort, high return.
Write down the assumptions in the transformation itself. A comment saying which values are expected and what happens to others is the thing the next person needs, and it is also what makes a review catch this.
Formalise it if the relationship supports it: an agreed contract on the fields you depend on, with a process for changing them. Worth it for a small number of critical feeds and overkill for everything.
And the incident habit: write this one up. Eight days of wrong numbers is exactly the kind of thing that gets forgotten in a fortnight, and the write-up is what turns it into the accepted-values test rather than into a story.