After looking over the Yamaha P-125 manuals
(especially the "Computer-related Operations" document), it looks like the Yamaha P-125 uses its USB TO HOST port for 2 or 3 simultaneous functions:
1. USB MIDI device
2. USB audio device
3. USB proprietary device(?) for song file transfers
When you connect the Yamaha P-125 to a computer, it will probably appear as a USB hub with 2 or 3 USB devices attached to it.
In the first review on this page
(the review by Koshdukai on 07.08.2021), the reviewer describes their Miditech USB MIDI Host worked with their Teenage Engineering OP-1 until a recent firmware update which upgraded that device to have both USB MIDI and USB audio functions. (That review also says if you attempt to use the Miditech USB MIDI Host with a device that sends MIDI Clock messages, it will hang after a few seconds. The Yamaha P-125 MIDI Reference document says it sends MIDI Clock messages.)
It looks like USB MIDI Host devices may often have issues with MIDI devices that act as USB hubs. When I looked at some other USB MIDI Host devices, some of them specifically say they have issues with devices that act as USB hubs, while others specifically mention they are compatible with devices that act as USB hubs:
• The Kenton MIDI USB Host
describes devices with a built-in USB hub will only work with its later firmware versions.
• The DOREMiDi MIDI USB Host
doesn't specifically describe compatibility with devices that act as USB hub, but its compatibility list says the Yamaha P-125 is compatible.
• The Sevilla Soft USB HOST MIDI 2
says it is compatible with devices that have an integrated hub.
• The HobbyTronics "MIDI USB to DIN Converter"
says it is not compatible with devices that have a built-in USB hub.