DNS resolvers cache answers for a period determined largely by the record's TTL. After you change a record, some users may continue seeing the older value until their resolver cache expires.
What to consider
Propagation is not a single global process that completes at one moment. Different networks and recursive resolvers can refresh at different times, and local device or browser caches can add another layer.
Before a planned migration, lowering TTL values in advance can reduce the time old records remain cached. After the change, test authoritative DNS and multiple external resolvers before assuming the update failed.