My troubleshooting approach would be something like this:
- Swap the Thunderbolt cables and test that for a while to see if it's possibly related to a cable.
- Then try swapping the USB-C ports that the monitors connect to, to see if it's isolated to a particular USB-C port.
- If the issue continues to be isolated to the "left" monitor, try moving all the connected USB peripherals to the "right" monitor, and see if that changes anything.
- If so, try unplugging each USB device one at a time (connect directly to the laptop if you absolutely can't live with out it) and see if you can isolate a specific USB device that might be causing the issue.
Basically, you're trying to isolate whether a cable, a USB device, or the monitor itself is at fault, and then you can focus your troubleshooting on the offending component.