Tinsa is by no means perfect but IMO it is the best of the indeces around. The official ones are laughable (according to them prices actually rose last year) and even people such as Kyero readily admit that theirs, based on asking prices, is not nearly as good as that of Tinsa.
Yes it is somewhat lagging behind the real market. What it does show well however are trends. In this case that property prices are dropping and that the rate of drop is accelerating.
If you, or anyone else for that matter, can suggest where I may find more accurate, publicly available, figures I’m open to suggestions.