If you search Google Maps for “Essex border” it shows a dashed red line around he border of Essex. However, do some reason Google has decided that Southend does not belong in Essex.
The last time I saw a more serious map error for Essex was when Google insisted on calling Danbury “Banbury”. Usually mistakes are minor, such as incorrect road names and places shifted half a mile up the road.
A search for the Kent border does not place Southend in Kent, so it appears to be nowhere. The Kent border search also expels the Isle Of Grain and Gillingham. At least it is not just Essex.
Not sure if any officials have yet noticed that loss of Southend. Royal Mail still recognise it as an Essex town so people living their can still rely on the postal service if not on Google.