Make sure you have the “Ignore permissions” check box active under the Syncthing folder’s Advanced tab. If Syncthing tries to apply permissions from a remote system, it will effectively erase all ACL-based permissions set on the Synology.
Overall this sounds like a general Synology permission problem. Have you looked at the parent folders? There is a permissions inspector tool within File Station, use it to check the effective permissions for the sc-syncthing user. When setting ACLs, use the sc-syncthing group, as recommended by the SynoCommunity documentation.