[OnePlus 9 Pro] fhortner:
The only option to connect your Oneplus 9 Pro to you Linux PC is to use a USB 2.0 cable. Once you use a USB 3.x cable, you run in the contradiction that the phone is connected via USB 3.0 but the device descriptor flag bcdUSB indicates only USB 2.0 is supported.

Once you are connected with a plain USB 2.0 cable you can establish a connection and verify the USB device descriptor under linux:

With the USB dialog on the smartphone not confirmed

lsusb -v -d 18d1:4ee8

which gives

Bus 001 Device 015: ID 18d1:4ee8 Google Inc. Nexus/Pixel Device (MIDI)
Couldn't open device, some information will be missing
Device Descriptor:
bLength 18
bDescriptorType 1
bcdUSB 2.00
bDeviceClass 0 [unknown]
bDeviceSubClass 0 [unknown]
bDeviceProtocol 0
bMaxPacketSize0 64
idVendor 0x18d1 Google Inc.
idProduct 0x4ee8 Nexus/Pixel Device (MIDI)
bcdDevice 5.04
iManufacturer 1 OnePlus
iProduct 2 SM8350-MTP _SN:XXXXXXXX
iSerial 3 XXXXXXXX
bNumConfigurations 1
Configuration Descriptor:
bLength 9
bDescriptorType 2
wTotalLength 0x0065
bNumInterfaces 2
bConfigurationValue 1
iConfiguration 4
bmAttributes 0x80
(Bus Powered)
MaxPower 500mA
Interface Descriptor:
bLength 9
bDescriptorType 4
bInterfaceNumber 0
bAlternateSetting 0
bNumEndpoints 0
bInterfaceClass 1 Audio
bInterfaceSubClass 1 Control Device
bInterfaceProtocol 0
iInterface 5
AudioControl Interface Descriptor:
bLength 9
bDescriptorType 36
bDescriptorSubtype 1 (HEADER)
bcdADC 1.00
wTotalLength 0x0009
bInCollection 1
baInterfaceNr(0) 1
Interface Descriptor:
bLength 9
bDescriptorType 4
bInterfaceNumber 1
bAlternateSetting 0
bNumEndpoints 2
bInterfaceClass 1 Audio
bInterfaceSubClass 3 MIDI Streaming
bInterfaceProtocol 0
iInterface 0
MIDIStreaming Interface Descriptor:
bLength 7
bDescriptorType 36
bDescriptorSubtype 1 (HEADER)
bcdADC 1.00
wTotalLength 0x0025
MIDIStreaming Interface Descriptor:
bLength 6
bDescriptorType 36
bDescriptorSubtype 2 (MIDI_IN_JACK)
bJackType 2 External
bJackID 1
iJack 0
MIDIStreaming Interface Descriptor:
bLength 9
bDescriptorType 36
bDescriptorSubtype 3 (MIDI_OUT_JACK)
bJackType 1 Embedded
bJackID 2
bNrInputPins 1
baSourceID( 0) 1
BaSourcePin( 0) 1
iJack 0
MIDIStreaming Interface Descriptor:
bLength 6
bDescriptorType 36
bDescriptorSubtype 2 (MIDI_IN_JACK)
bJackType 1 Embedded
bJackID 3
iJack 0
MIDIStreaming Interface Descriptor:
bLength 9
bDescriptorType 36
bDescriptorSubtype 3 (MIDI_OUT_JACK)
bJackType 2 External
bJackID 4
bNrInputPins 1
baSourceID( 0) 3
BaSourcePin( 0) 1
iJack 0
Endpoint Descriptor:
bLength 9
bDescriptorType 5
bEndpointAddress 0x01 EP 1 OUT
bmAttributes 2
Transfer Type Bulk
Synch Type None
Usage Type Data
wMaxPacketSize 0x0200 1x 512 bytes
bInterval 0
bRefresh 0
bSynchAddress 0
MIDIStreaming Endpoint Descriptor:
bLength 5
bDescriptorType 37
bDescriptorSubtype 1 (Invalid)
bNumEmbMIDIJack 1
baAssocJackID( 0) 3
Endpoint Descriptor:
bLength 9
bDescriptorType 5
bEndpointAddress 0x81 EP 1 IN
bmAttributes 2
Transfer Type Bulk
Synch Type None
Usage Type Data
wMaxPacketSize 0x0200 1x 512 bytes
bInterval 0
bRefresh 0
bSynchAddress 0
MIDIStreaming Endpoint Descriptor:
bLength 5
bDescriptorType 37
bDescriptorSubtype 1 (Invalid)
bNumEmbMIDIJack 1
baAssocJackID( 0) 2

With the USB dialog on the smartphone already confirmed

lsusb -v -d 22d9:2764