Data transfer over USB becomes unreliable after upgrading to v5.4.83-v71+

Start with the USBdataTest.zip reproduction, including lanTestPi2.py and lanClientTest.py, after configuring dwc2 and g_ether in /boot/config.txt and /boot/cmdline.txt. Compare repeated transfers on Linux v4.19-v71+ and v5.4.83-v71+ using the supplied /etc/network/interfaces file. Done means the regression is isolated and a confirmed fix or workaround is documented.

I think there is a bug in the latest pi4 Linux release, because data transfer over USB becomes unreliable after upgrading from Linux v4.19-v71+ to v5.4.83-v71+ on raspberry pi4.

We are using the USB C port as a RNDIS gadget to get TCP data to a PC. I found the minimum python code required to reproduce the problem, it sets up a socketserver on the pi4, and the client on the PC repeatedly asks for 10MBs data. In Linux v4.19-v71+ it is robust, and I can do the transfer > 5000 times (expected behaviour). In v5.4.83-v71+ exactly the same code works 50 to 500 times then the link starts to break down, I get time out errors (even with time out set to 10 seconds), and frequently after that the link freezes entirely and requires the USB to be disconnected and reconnected for it to return. It even takes down an SSH connection to the pi which is separate software (but connecting over the same USB cable).

I'm unable to use v4.19-v71+ from now because it is not compatible with the latest pi4 hardware, so I hope someone can help - either with a bug fix or workaround suggestions.

To reproduce:
Test code I am using is here:
USBdataTest.zip

Set up the pi4 as a usb gadget:
To /boot/config.txt add:
dtoverlay=dwc2
To /boot/cmdline.txt add:
modules-load=dwc2,g_ether
and use the /etc/network/interfaces file provided.
On the pi4 run:
python3 lanTestPi2.py
On a windows PC run:
python3 lanClientTest.py
It can be reproduced with or without an external PSU for the pi.

Other information:
PC OS: Windows 10
Pi4 boot loader: Sep 3 2020
Python version: 3.7

• Which model of Raspberry Pi? Pi4
• Which OS and version? 175dfb027ffabd4b8d5080097af0e51ed9a4a56c, stage2
• Which firmware version?Jan 8 2021 14:32:15 version 194a85abd768c7334bbadc3f1911c10a7d18ed14 (clean) (release) (start_x)
• Which kernel version? Linux TupelPi1 5.4.83-v7l+ #1379 SMP Mon Dec 14 13:11:54 GMT 2020 armv7l GNU/Linux
  I upgraded by using:
  sudo apt update
  sudo apt full-upgrade

Logs:
Time out error (from PC recv command).
No error from pi4, but PC can no longer connect to it.

Please let me know if there is any other information I can provide, or if I can do any tests for you to narrow down the problem.