Disappearing trade route

During my current Maratha campaign, I've had trade relations with the Ottomans since Turn 1. They're at war with the Persians who periodically raid/blockade Basra cutting off the trade route (bright green route showing positive income turns dark green with zero income).

Suddenly, around 1735 or so, the trade route disappeared. My diplomatic screen shows that I still have a trade agreement in place with the Ottomans, but the trade screen shows no trade income with them, and the trade route doesn't show on the map, not even in its muted 0-income dull green form.

Any idea what caused this?

If their capital (Istanbul) is taken all their trade in now redirected to an other port. This is usually the port in Anatolia. For some reason, the game doesn't reset the trade route and human players loose their trade route. This is a bug. Ending the trade agreement and getting a new one can solve the problem.

That would seem to be it; my Thugee just walked past Istanbul where there is now an Austrian flag flying.

Thanks.

On a side note, the same Austrians, despite having taken all of the Ottoman territories in Europe, are lagging severely in the prestige department.

Most likely because their naval prestige is very low, same reason why Russians rarely are in top 5 unless played by human.