When attempting a manual VM datastore migration in a datastore cluster using Storage DRS the "Disable Storage DRS for this virtual machine" option is enabled by default.
VMware vCenter Server
This behavior is intentional and expected.
When the user chooses to migrate a virtual machine to a different datastore within the datastore cluster, Storage DRS acknowledges this as the user's decision, assuming the user has more knowledge about the situation. The design behind this is that when a datastore is selected, the user is indicating that this datastore is the optimal choice, meaning the user has information that Storage DRS may not have.
To prevent future migration recommendations to other datastores, Storage DRS is disabled, ensuring the virtual machine remains permanently placed on the selected datastore.