As @k6ccc explained: it should be running fine. If VLANs are configured right, then internet uplink device will only see MAC address used by corresponding VLAN on your OpenWRT router. L2 switches don't change MAC addresses, used for communication between ethernet devices.
@k6ccc @mkx
Thank you both for your replies and attentions to this post! And I can completely understand your suggestions. However, my scenario is a little bit different.
Actually the main issue here is about "switch detection" from uplink. As I mentioned in my post, the uplink cannot be connected to a switch, otherwise this link is simply disabled. I even tried it with an unmanaged switch without any devices attached to it, and that link is still disabled. If I connect this link to the Ethernet port on my computer and configured with correct IP address, it's enabled after about 15 secs. I think the uplink is controlled by a Cisco L3 switch, which supports "switch detection" or access-port only options.
So my root question is: is it possible for SwOS to bypass this restriction? Any input is appreciated!