Like with a large and growing list of conflicts, this one goes back to the definitions of the words.
Let’s define doctrine.
From the 1828 dictionary: In a general sense, whatever is taught. Hence, a principle or position in any science; whatever is laid down as true by an instructor or master. The doctrines of the gospel are the principles or truths taught by Christ and his apostles.
In that sense, both policy and unchanging truth are doctrines of the church. An official doctrine just means an official teaching. Whatever the church teaches officially at the time it is taught is the doctrine of the church. It doesn’t have to be permanent to be true doctrine, and it doesn’t have to be forever out of the question to currently be false doctrine.
If someone thinks that doctrine is unchangeable, they don’t understand what doctrine is.
Because doctrine is simply the official teaching, doctrine is always changing as the people progress. What doesn’t change is God and His Eternal nature, His character, His nature, His principles, and His work and glory. He is the unchanging one.