Q. Are Catholics required to follow the Pope's teaching on moral issues, or only his teaching on doctrine?
All doctrinal teachings involve the issues of faith and morals. Although every encyclical letter addressed to the world on matters of faith and morals in not an infallible pronouncement, if the Pope restates a long-held teaching of the Church (for example, the evil of artificial contraception or that priestly ordination is reserved to men) a good case can be made for the infallibility of that restatement, even if the Pope does not specifically identify his encyclical as infallible, since he is definitively proclaiming that a certain doctrine of faith or morals is to be believed.
Catholics cannot disregard non-infallible statements from the Pope or their Bishop. Divine assistance is given to the Pope and the bishops in communion with him when they, as part of their ordinary teaching duties, propose a teaching that leads to a better understanding of divine revelation in matters of faith and morals. The faithful are to submit humbly and adhere sincerely to the teachings of the Pope, even when he is not speaking infallibly, because his ordinary teaching is an extension of his extraordinary charism.
Recommended reading:
● Catechism of the Catholic Church, paragraph 892