You've been moved to a 21CN/WBC/WBMC/IPSC connection, and for a long time there has been a problem of this kind with that and the BT speed test. It used to be reported frequently on these forums, then died away, and has re-surfaced in the last few weeks.
It looks as though it was fixed, and someone has broken it again
In your case, it's almost as though the IP Profile has simply been ignored. What does your Plusnet Control Panel show as the Current line rate
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