When I go to the observatory and buy some prophecies just after a quest echo, I lose around 30 stability.
A few tests I did:
- Buying one or more prophecies after a Quest Echo: about 30 stability loss. Reproduced 3 times, on 2 different characters.
- Buying prophecies after a normal echo: stability unchanged.
- Going to the observatory after a Quest Echo, but not buying anything: stability unchanged.
- Going anywhere else than the observatory after a Quest Echo: stability unchanged.
Hope this helps.