Unfortunately I do not have a clear answer for this as my ISP resolved the issue. The ISP provider said there where issues with the switches withing the data centres. Even though the BGP links were up on their side and showing correct confirmations and speed, they acknowledged that several properties in the area (hundreds apparently) were experiencing symptoms similar to mine. The issue took 3 weeks to be resolved.

If I run the iperf3 tests now, I get text book output with similar bitrates across both single and multiple stream tests.