That's correct, Selective Copy should not be picking up incrementals for MS SQL. The only exception is for "Selective Copy for Oracle, Oracle RAC and SAP for Oracle agents" -> where an online selective full is required
Documentation - https://documentation.commvault.com/commvault/v10/article?p=features/storage_policy_copy/selective_copy.htm
In you scenario, that does not look like the case.
Can you please confirm whether you have been modifying the setting (in the properties of Secondary Copy) under the Tab "Copy Policy" "All Backups" to an older date?
Furthermore if this issue is concerning, I do recommend to open a ticket with Support