I've lost a lot of time trying to understand why the package updated package (already in the repo) didn't compile.

My conclusion is (reported here since it can be useful for future packages):

dh_clean (seems to) starts before quilt patches are applied.

In this specific case the error, fixed via quilt patches, occurred in the "clean" phase, that runs before the patches are applied (a strange situation).

So i just disabled the "clean" phase, adding:

override_dh_auto_clean:
override_dh_clean:

to debian/rules, and the package now build correctly.