On Chinese Wiki, this is marked as UI display bug. This mechanism is very similar to civ5.
Once siphon funds succeed, the thief steals 8 times the current GPT of the target city. In next 8 turns, victim will suffer gold lost every turn, which value equals to this current GPT of this city, but won't display on the UI (so called display bug). Not sure if it is correct, just wiki.
This bug is actually quite vicious. At least I had some experience, I didn't notice that I was stolen by AI and only focused on the GPT on the top UI, which led me to bankruptcy. Great Zimbabwe is easy to cause a 5000+ gold lost, and hard to make up for it - this city often provides 75%+ of your national income and now the remaining 25% is for upkeep, which leads to actual negative growth...
If you've ever used Cree, you'll find that spies from the entire world will all run to your Zimbabwe city to steal money. So if you want to build Great Zimbabwe, then at least put two spies in that city (1 is not enough, I've been stolen by 3 AI players in the same turn and caught 1, the rest 2 succeed. This city produce 720 GPT so I lost 5760 * 2 = 11520 gold and apparently I'm broke).