FTTC upstream rate plummeted

For the last 3,4 (maybe longer) years, my parents have had a fairly reasonable sync speed, 38-42 down, 7-8 up.

In April this year, there was a re-sync and the upstream was around 4.5. I put this down to some freak noise event, and later in the day re-synced but then it only improved to just under 6, which was annoying but not really worth doing anything about.

This is the bit loading from April after it degraded the first time
https://ibb.co/qr5hYZR

Then on Saturday came another re-sync in the middle of the day, this time to a paltry 2.6 upstream, whilst the downstream is still fine at 40. I've tried a couple of re-syncs since but the upstream hasn't budged from 2.6.

This is the bit loading now
https://ibb.co/44dwYKP

You can see the 2nd upstream band has been suppressed compared to April. My firsts thoughts were a noise source but then I'd expect to see more of a bandpass affecting the surrounding downstream bands but they don't seem to have changed noticeably. The upstream margin is still 6 dB as it was in April (downstream is 3dB)

This is the dsl checker output, so the downstream is performing better than expected whilst the upstream is half the expected minimum.
https://ibb.co/SvKFXfY

Even stranger, I noticed that today the NTEFaceplate line has changed from 'N' to 'P'. When i checked at the weekend with test date 03-07-2019, it was still 'N', as it always has been.

According to https://www.ispreview.co.uk/index.php/2017/07/bt-who... 'P' would imply no voice service on the line, which there is...

Any thoughts?

Checker does nothing in terms of changing service profile

If on an ECI cabinet it could just be another couple of customers going live and causing cross talk impacting things

I got the impression that info was gathered passively from the checker, so what could have triggered it to change to 'P'?

The cab is Huawei and modem an HG612

A finger problem when updating other data in the checker - the Wholesale checker is known for behaving oddly at times.

Huawei so vectoring should be helping, one possibility is that someone has connected a modem locally that does not support vectoring and is trashing things, hence why Openreach when they spot these switch those lines off and inform ISP I believe - though that is rare

Could be someone on the cabinet using 1 of the rogue Asus modems.
They allow the user to disable UPBO (Upstream Power Back Off) which can cripple the upstream of other lines.

My Huawei HG612 randomly dropped from 8Mb upstream to 4Mb upstream one day.
The downstream wasn't affected.
It just wouldn't sync back at the normal upstream rate for no reason.
Replacing it with a Zyxel VMG8924-B10A immediately fixed the issue.

I later replaced the 8924 with a Zyxel VMG1312-B10A which increased the upstream by another 1Mb, giving me around 9Mb.

What modem are you using? Have you tried another modem on the line?

My FTTC upstream was 11Mb a few years ago.
Crosstalk has knocked this down to its current rate of 7.3Mb.

It could just be crosstalk, could be your modem, could be a misbehaving modem on the DSLAM.

Try another modem.
If that doesn't work try get a port swap on the DSLAM.