Shanghai Mint - An 'official-unofficial' addition to the Classical Garden Series?

[ArthGoch]

 Between 2010 and 2016 the Shanghai Mint produced a series of nine 2 oz medals based around the subject of the Classical Gardens of China. These were available in a number of metals/finishes and with varying mintage runs. In 2017 the series concluded with a monster 20 oz silver medal and with the mint confirming that the series had finished

However it appear that a 10th medal was struck ( or at least the design was completed) and with this medal released at the end of last year and post the 'official' completion of the series, 

 This medal is based on the water garden of Rugao Shuihui and  currently the mintage for these is towards the low end of the spectrum for the series; ( for example antique/matt is showing at 200) 

I know that in the past forum members have collected this series - does anyone have any credible reason to why this official-unofficial medal has now been released and similarly why was this not released at some point during the official period of the series?

Cheers!