The difference between your second and third links appears to be only the regular spikes. Plusnet have "always" had those on certain pathways through their gateways. As far as I know nobody has ever reported performance issues due to them.
The large jump between your first two, (I notice the base average latency at the start of the second is already considerably higher than that in the first), is a different question. I suspect the rise you aren't showing will be either interleaving kicking in for some reason or being swapped to the new network. A link to that might give a clue
If you were on 40/20 the disconnections on your third will almost certainly be Plusnet trying to apply the new profiling to simulate 55/10. As a by-product making things worse wrt latency.
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