MT7688: ethtool says "no such device" for eth0


I'm trying to figure out why ethtool always reports "Cannot get device settings: No such device", altough it can display the message level and link status:

$ ethtool eth0
Settings for eth0:
Cannot get device settings: No such device
	Current message level: 0x000000ff (255)
			       drv probe link timer ifdown ifup rx_err tx_err
	Link detected: yes

...while the eth0 interface is working perfectly ok:

$ ip link
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP qlen 1000
    link/ether 40:a3:6b:c0:78:cb brd ff:ff:ff:ff:ff:ff
3: wlan0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN qlen 1000
    link/ether 40:a3:6b:c0:78:c9 brd ff:ff:ff:ff:ff:ff

As far as I can see the mtk_soc_eth driver has support for ethtool ioctls (SIOCETHTOOL with commands like ETHTOOL_GSET etc.).

From the error message it seems ethtool cannot even find the eth0 device.

Any idea what I'm doing wrong here?

This is on OpenWrt 22.03.2, on a Onion Omega2 (MT7688) module.