If I had to guess the symptoms look kind of crosstalk related. However that is one heck of a hit to take all at once.
Your downstream SNRm went from 7.4dB to -2.8 (10.2)
Your upstream SNRM went from 13.2dB to 6.3dB
The upward spike is interesting, thats the sort of thing you could expect to see if a whole lot of users suddenly got knocked offline and had to resync. Some of the BCM chipsets such as those used by the HG612/Zyxel seem to come back up much faster than say the HomeHub5a.
However the corresponding spike in your max attainable doesnt quite give the headroom I would have expected if your downstream SNRm really was at 14dB so Im not quite sure exactly what happened there.
The spikes indicate that what ever affected you, also had an impact on some of your neighbouring lines.
19:38 is a bit of an odd time for a new disturber to appear, but it is possible that someone got FTTC swapped over during the day but it was a self install and they didnt switch their modem on until that time when they got home from work.
What could have happened if your modem is happy doing its thing, neighbour turns on their modem. Bam crosstalk which immediately makes your SNR drop to minus 2.8dB. The line cant hold at such a negative figure so attempts to resync at the target of 6dB which accounts for your loss of speed.
Although its not unusual for crosstalk to suddenly knock 10Mb from a lines max attainable, I dont think Ive seen what looks like one single disturber take 20Mbps off a line that was previously syncing at around 60Mbps.
This isnt anything to do with Plusnet configuration, they have no control over your SNRm or sync speed (other than provision type ie 80/20,40/2). Such a drop does however need investigating so you will have to go through the fault process
----
PS note for others. Ive not looked at all stats or params. Only concentrated on main figures and not any config params.
QLN is a bit messy... some noise... and also upstream crosstalk. Slight signs of crosstalk bump at tone 3500 onwards. Noise in D1 & D2 makes it a bit impossible to spot any obvious crosstalk in those carriers.
Hlog looks fine indicating physical line ok. Slight bump also around 3500 but not anything Im too concerned about.
Attaching zoom of SNRM
Im only able to type one handed atm so its taken me a while to make this post and I need to get to bed myself.