syslog shows a bunch of FF messages

The OnePlus 7 Pro port is named "guacamole". You can find the source code at LineageOS/android_device_oneplus_guacamole. You may want to search for the error here, but if you can't find it check inside the setup-makefile.sh, you'll find the line,

export DEVICE_COMMON=sm8150-common

That means the kernel's source code is located LineageOS/android_kernel_oneplus_sm8150. You can also find this link on the in the documentation page for the port under "kernel link".

If you do a search in the LineageOS/android_kernel_oneplus_sm8150 repository for "GAD SUS" in that repository you'll find the source of this error which is drivers/usb/dwc3/gadget.c:3654. So the problem is in USB Dual Role Device, which is an ASIC driver described as,

The Synopsys DesignWare® USB 3.0 Dual-Role Device (DRD) Controller is a set of synthesizable soft IP that ASIC/FPGA designers can use to implement a complete USB 3.0 DRD for 5 Gbps speeds. The USB 3.0 DRD Controller is compliant with the specifications for SuperSpeed, High-Speed, Full-Speed, and Low-Speed USB speeds. Optimizing development time and minimizing engineering risk, the DesignWare USB 3.0 DRD Controller helps designers bring their host and peripheral designs to market faster for wired and wireless applications such as smart phones, tablets, TVs, set-top boxes, cameras, modems, game consoles, PCs, and telecommunications equipment. In addition, the IP is silicon proven in design process nodes from 180-nm to 7-nm.