I wok in the electricity supply industry, and while I can't offer a technical opinion, it might be useful to consider the 50Hz grid frequency number from the perspective of why it exists at all.
The frequency of supply is, necessarily, dynamic. It needs to change in response not only to the different magnitude and nature of load, but also to the way in which that load is supplied. For that reason, it was never intended as a frequency standard - that is an incidental use which has been made of the electricity supply. There are alternative and far more accurate frequency standards designed for that purpose.
The grid code uses frequency to control the grid.There is a 'Target Frequency' but that's only one definition in the code. More important are the specs for changes of frequency, both in rate and magnitude, which trigger different actions for both those who put power on and take power off the grid. Those actions ensure that power is available to all in adequate quantities exactly when required.
The grid code is an unwieldy document, and it sets very stiff obligations on commercial operators who supply power to the grid - this has arguably become more important in recent years when power is supplied by a larger number of smaller generating units.
In short, I would argue that grid frequency is not there to be a standard in itself, but as a tool to enable the grid to do its job.
Using the grid frequency as a standard for timekeeping means accepting the grid code (ie nominally 50Hz, controlled between 49.5Hz -50.5Hz) and the error implicit in that.
The grid code sets acceptable frequency ranges but there are no obligations to put extra cycles in (or take extra cycles out) so that an exact number of cycles are delivered over a set period of a day, a week or a year. To do so would be utterly contrary to efficient, stable electricity supply.
Synchronous clocks might mean we can record that average grid frequency has tended below a nominal 50Hz over a period, but that doesn't mean that there has been a failure to keep to the standards set by the code. There is a reporting mechanism on grid transactions at:
https://www.bmreports.com
This is where system notifications are posted in the event of forecast shortfalls in power etc.
For interest, a couple of extracts from the code:
Grid Code Definitions-
Target frequency :That Frequency determined by NGET, in its reasonable opinion, as the desired operating Frequency of the Total System. This will normally be
50.00Hz plus or minus 0.05Hz, except in exceptional circumstances as determined by NGET, in its reasonable opinion when this may be 49.90 or 50.10Hz. An example of exceptional circumstances may be difficulties
caused in operating the System during disputes affecting fuel supplies.
Grid Frequency Variations
CC.6.1.2 The Frequency of the National Electricity Transmission System shall be nominally 50Hz
and shall be controlled within the limits of 49.5 - 50.5Hz unless exceptional circumstances prevail.
CC.6.1.3 The System Frequency could rise to 52Hz or fall to 47Hz in exceptional circumstances.
Design of User's Plant and Apparatus and OTSDUW Plant and Apparatus must enable operation of that Plant and Apparatus within that range in accordance with the following:
Frequency Range Requirement
51.5Hz - 52Hz Operation for a period of at least 15 minutes is required each time the Frequency is above 51.5Hz.
51Hz - 51.5Hz Operation for a period of at least 90 minutes is required each time the Frequency is above 51Hz.
49.0Hz - 51Hz Continuous operation is required
47.5Hz - 49.0Hz Operation for a period of at least 90 minutes is required each time the Frequency is below 49.0Hz.
47Hz - 47.5Hz Operation for a period of at least 20 seconds is required each time the Frequency is below 47.5Hz.
For the avoidance of doubt, disconnection, by frequency or speed based relays is not permitted within the frequency range 47.5Hz to 51.5Hz, unless agreed with NGET in accordance with CC.6.3.12.