Adding SO2 (campden tablets) before fermentation results in extra
acetaldehyde in the product. This means that unless you add even more
when fermentation finishes your wine will be susceptible to bacterial
attack.

Your SG seems rather high for a finished fermentation. It is possible
that the combination of alcohol plus residual sugar has reached the
tolerance level of your yeast.
During fermentation one molecule of sugar produces two molecules of
alcohol. This means that the water activity of your wine decreases
(osmotic pressure increases) as fermentation proceeds. With a
suficiently high initial sugar content the limiting water activity of
your particular yeast can be reached before all the sugar has been
fermented and fermentation then dies. I have seen this happen on
ferments of over 20,000 litres. A bit embarrasing but addition of 10%
water would usually restart the ferment.
I did include this in my book on beverage chemistry a couple of years
ago.

Bob Molony
www.molab.co.nz