Originally Posted by
tonynz
On the app it says that I have already achieved 600 TP to maintain. Wonder if that's a bug.
It may just be telling you that having earned 600 TPs in the current membership year, that has been enough to maintain Silver in the next (2017-2018) membership year - which is true.
To maintain Silver beyond that and into 2018-2019, you must earn at least 600 TPs and have taken 4 qualifying flights between 9 November 2017 and 8 November 2018.