The problem is there from DC at "Zero Hz" and upwards.
Consider a simple DC circuit such as a battery feeding a load.
Maximum power transfer occurs when the load equals the internal resistance of the battery; and the potential voltage of the battery is shared equally over that internal resistance and the load.
With a typical battery potential of 1.5V, at maximum power transfer, the load has 0.75 Volts across it, the other 0.75 Volts being apparently "lost" across the internal resistance of the battery.
Now disconnect the load totally, no power out, so the terminal voltage of the battery rises to "equal" the potential voltage of 1.5V
WARNING - DO NOT DO THIS AT HOME.
Now short-circuit the battery, say by replacing the original load with a simple piece of wire.
The voltage across that short-circuit wire is 0 V (zero volts), with the potential voltage of the battery being totally lost inside the battery on its internal resistance.
Analyse the three conditions, assuming the DC to be an extremely low frequency AC.
The pattern fits, as does testing at intermediate loads, from open circuit down through matching load to short-circuit., similar to partial reflections.
Given the lengths/distances of the National Grid, similar can happen on that; and is also part of the reason for the Import/Export Interconnectors handling DC only, as well as the more obvious Frequency and Phasing aspects.