The  DT node in [2] is probed by realtek_smi_probe() [3]. The call flow is:
realtek_smi_probe()
  -> dsa_register_switch()
    -> dsa_switch_probe()
      -> dsa_tree_setup()
        -> dsa_tree_setup_switches()
          -> dsa_switch_setup()
            -> ds->ops->setup(ds)
              -> rtl8366rb_setup()
                -> realtek_smi_setup_mdio()
                  -> of_mdiobus_register()
                     This scans the MDIO bus/DT and device_add()s the PHYs
          -> dsa_port_setup()
            -> dsa_port_link_register_of()
              -> dsa_port_phylink_register()
                -> phylink_of_phy_connect()
                  -> phylink_fwnode_phy_connect()
                    -> phy_attach_direct()
                       This checks if PHY device has already probed (by
                       checking for dev->driver). If not, it forces the
                       probe of the PHY using one of the generic PHY
                       drivers.

So within dsa_register_switch() the PHY device is added and then
expected to have probed in the same thread/calling context. As stated
earlier, this is not guaranteed by the driver core.

Have you looked at:

commit 16983507742cbcaa5592af530872a82e82fb9c51
Author: Heiner Kallweit [off-list ref]
Date:   Fri Mar 27 01:00:22 2020 +0100

    net: phy: probe PHY drivers synchronously

See the full commit message, but the code change is:

iff --git a/drivers/net/phy/phy_device.c b/drivers/net/phy/phy_device.c
index 3b8f6b0b47b5..d543df282365 100644

--- a/drivers/net/phy/phy_device.c
+++ b/drivers/net/phy/phy_device.c

@@ -2577,6 +2577,7 @@ int phy_driver_register(struct phy_driver *new_driver, struct module *owner)
        new_driver->mdiodrv.driver.probe = phy_probe;
        new_driver->mdiodrv.driver.remove = phy_remove;
        new_driver->mdiodrv.driver.owner = owner;
+       new_driver->mdiodrv.driver.probe_type = PROBE_FORCE_SYNCHRONOUS;
 
        retval = driver_register(&new_driver->mdiodrv.driver);
        if (retval) {

How does this add to the overall picture?

    Andrew