There are 5 layers of problem here:
1) The actual VDSL2 behaviour on the line.
2) The reporting by the ASUS, at minimum, and possibly its behaviour.
3) The copy of the IP profile in Plusnet (visible as "current line speed").
4) The reporting of the IP Profile in BT (visible in BT speedtests).
5) Plusnet performance when everything is correct.
It looks like you have had problems with all of them, that might be contributing to the current issue
1) Your line is currently syncing at the highest possible speed: 79998/19999 is close enough to 80/20, with plenty of spare headroom.
I suspect that when Plusnet shows "78Mb" as the "current line speed", it means it hasn't received an update from BT. My line (with an 80/20 sync, and a similar amount of headroom), currently shows "77Mb" - which I thought was the standard value when things worked OK.
Some of the other peices of information suggest that the sync speed hasn't always been 80/20.
2) The ASUS is reporting something wrong about the state of your line. It seems to be both reporting "Fastpath", which only happens when DLM has set "INP=0" and "delay=0". However, your ASUS is reporting non-zero INP values both upstream and downstream, and both are non-standard.
In addition, your ASUS is reporting that your current downstream sync is 80Mbps with a maximum attainable of 102Mbps - yet it reports the SNRM to be only 6.5dB. That isn't consistent; to get an attainable of 102Mbps would require an SNRM of around 11.5dB.
Something is a off in the reporting there, and (like @j0hn), I have something of a distrust for current ASUS VDSL2 modems. It is hard to figure what they are doing.
In particular, it seems to be hard to figure what DLM has done to the line.
3) The plusnet copy of the "IP Profile", the "current line speed" has seen plenty wrong, starting with the value of 21 when the sync speeds were wrong.
However, seeing this go from 78 to a value of 72.8 suggests that the sync speed has been something different from 80/20 recently. And that the process that copies the IP profile from BT to Plusnet has caught a drop in speed, but has missed the subsequent increase in speed.
One cause of a drop from a full 80/20 sync is when DLM imposes interleaving and FEC correction, which can steal ~ 10% of your speed. We have started to see this more and more after a line is given a DLM reset ... and one of those now seems to happen as part of a migration. It is getting common to see someone complain "I just migrated and lost some of my speed" ... and you may have done the same. *If* that happened, then there's a good chance that DLM changed the settings again 2 days later.
So ... I might suspect that your "migration" caused a DLM reset, which caused some speed changes, which caused some IP-Profile changes, which caused some "current line speed" changes. But it isn't clear and obvious, because I don't trust the ASUS data (or behaviour), and I'm not 100% sure whether you have told us both values for the "IP Profile" (as BT knows it) *and* the "current line speed" (as Plusnet's own copy of BT's IP profile).
HOWEVER.... If the ASUS is correct that your attainable is 103Mbps, your line should easily have absorbed any extra overhead, and retained an 80/20 sync ... so there wouldn't have been a sync change or an IP profile change. On the other hand, if the ASUS is right with an SNRM of only 6.5dB, then the line wouldn't have had enough spare capacity. The ASUS is making it hard to know what figures to trust, so making it hard to predict behaviour.
If Plusnet has missed a report of an increase in IP profile, there are 2 ways to attempt to trigger an update:
a) Stop and restart the PPP session within the router. I can't advise on how to do this on an ASUS
b) Power cycle the whole modem/router. The best practice for doing this, to avoid DLM intervention, is to leave the modem/router powered down for 30 minutes.
After this, you should get the "current line speed" to 77Mbps (if your sync speed is truly 80/20). If you can't trigger a change yourself, you might need to ask Plusnet support to change it manually - either by phone, or by PN's forum.
4) BT's own copy of the IP profile is most easily accessed via their speedtester
http://speedtest.btwholesale.com/and following the "extra diagnostics" path.
While Plusnet have set my "current line speed" to 77Mb, my "IP Profile" reported by the speedtester is 77.35Mbps
(I'm not sure you have told us this value, and have instead reported the Plusnet value. If I'm wrong, then I apologise.)
However, BT appear to hold two copies of the IP Profile internally, and only one of them gets displayed to you. It has been known for BT to end up with a mismatch internally, which can cause the issue you have seen.
5) Once everything is corrected, your download throughput is likely to be around 74Mbps on a Plusnet line. This is a couple of Mbps below the peak available to other ISPs (such as BT Retail). It looks like their method of running QoS causes a loss of the very top slice of performance, but this has never been officially acknowledged.
