This relates to issue 3570335 which was apparently badly closed: in theory one could use STM32_ETH1_CHANGE_PHY_STATE to prevent the problem, however, there is still a PHY powerdown MII write in mac_lld_init which is not disabled by this #define.
The fix is trivial: just surround the mii_write at the end of mac_lld_init in os/hal/platform/STM32/mac_lld.c with #if #endif as follows:
/* PHY in power down mode until the driver will be started.*/
mii_write(ÐD1, MII_BMCR, mii_read(ÐD1, MII_BMCR) | BMCR_PDOWN);
On a side note, I find the STM32_ETH1_* naming a bit confusing since STM32_MAC_* is used elsewhere for anything related to the STM32 MAC driver.