@TP-Link 

Thanks for your prompt response.  Please find additional information below:

For your case, we might need to collect some basic information and forward to the senior engineers:

1. What is the model number of the Telecom router?

Speedport Pro Plus  Model Number: 40823109

2. Would the internet work fine if we did not process the speed test?

Internet works fine without the AV1300, i.e. purely through the router.  With the AV1300, it drops in and out, potentially with high data flow.  

3. What is the firmware version of the TL-WPA8631KIT V3?

I upgraded the Firmware to the latest:wpa8631pv3_eu-up-ver3-0-0-P1-20200927-rel61199-APPLC

Same (speedtest) problem unfortunately.

4. The speed test proceeds smoothly without the powerline adapters unplugged in, right?

Correct.  The speed test works fine with the router on its own. 

Additional info:

With the powerline adapters The TP link utility is showing c. 250Mps (i.e. Strong) between the adapter and extended. so I'm really confused.

@vipercity 

I have similar problem. The PLC MIMO noise causing massive interference to VDSL2 modem data line. Erratic download performance and high ping latency (> 100 ms) after the Powerline adapter plugs into any socket within the house wiring system. Disable MIMO (Lower PLC-to-VDSL interference mode) reduces the interference but at same time cut PLC connectivity speed dramatically. I reckon Powerline MIMO is more suitable for Fibre to house Internet. Just my 2 cents.