I have no idea. Maybe something to do with not actually using the same collation provider?
The freebsd package contains modifications to make icu work, or at least did. Possibly, you just stopped using the same collation provider, without even realizing it, because they switched over to a new provider without accounting for pg_upgrade. If so, then that's a bug in the package. This is a total speculation, but makes a certain amount of sense to me.
