The issue was that I had three separate subscription products (e.g., one month, three months, and one year), and users were switching between them. However, for the billing portal to handle subscription updates correctly, the subscriptions need to be part of a single product with multiple pricing tiers.

The correct approach is to have a single subscription product with all pricing tiers (e.g., one month, three months, one year) associated with it. Users should be allowed to change prices within the same product. If a user changes to a different product, the billing portal will process the change immediately instead of applying the correct "schedule at period end" behavior.