S3 External Account ACL Disappears

My company has two AWS accounts and syncs files periodically between S3 buckets in the different accounts. We do this by having an ACL entry on the bucket with a grantee of the external account canonical ID of the other account. This works fine initially, but when I try to sync files again (via AWS cli) a few weeks later, this ACL entry has disappeared and files can't be synced. This has happened several times now, over the past year or so. How can I find out what's causing the ACL entry to disappear and prevent that from happening?