Actions like that have caused great problems in the past, as the
kernel won't allow channel control of a monitor interface at all when
there is a managed interface on the same phy (afaik).

But just for fun and codepath testing here are two test scenarios:

Test 1: "iwconfig t2u mode monitor"
Sees nothing on any channel, no packets reported

Test 2(requested test): "iw phy phy11 interface add t2uhmon type monitor"
Sees nothing on any channel, no packets reported.
Despite having a monitor and managed interface on the same phy,
iwconfig is reporting that the channel is changing as requested.  So
perhaps my above "afaik" comment is partially outdated.

For both tests the interface was not used to connect to any ap prior
to or during testing.

Could you please try following patch?
Regards,

Lorenzo

--- a/drivers/net/wireless/mediatek/mt76/mt76x0/phy.c
+++ b/drivers/net/wireless/mediatek/mt76/mt76x0/phy.c

@@ -756,11 +756,10 @@ __mt76x0_phy_set_channel(struct mt76x0_dev *dev,
 
 	/* Vendor driver don't do it */
 	/* mt76x0_phy_set_tx_power(dev, channel, rf_bw_band); */
+	mt76x0_vco_cal(dev, channel);
 
 	if (scan)
-		mt76x0_vco_cal(dev, channel);
-
-	mt76x0_mcu_calibrate(dev, MCU_CAL_RXDCOC, 1);
+		mt76x0_mcu_calibrate(dev, MCU_CAL_RXDCOC, 1);
 	mt76x0_phy_set_chan_pwr(dev, channel);
 
 	dev->mt76.chandef = *chandef;