Some other notes: looking at the trade map, I can see that the only source of iron is from the same province as 3 horses. I'm already trading for two of those horses, and that trade is not being interrupted. Neither is the silk, which comes from the same province and follows the same trade route. And yet the iron will suddenly disappear but the other resources do not.

And as the final insult to injury, I ran a test where I purchased 1 copper (that I didn't need) and 1 iron. This time the purchase stuck. I went to the next turn and still had my iron (and extra copper), so I purchased 3 more, which also stuck. I then went to the next turn and lost all 4 iron. Still have the 1 copper though. The iron resource is literally located in the hex next to their horses, and follows the same trade route. I've been trading horses with this empire with no problem, but the iron just keeps disappearing. I can see the iron extractor and there are no armies on or near it.

Bumping this as I have encountered a similar or the same issue. From what I can see it was reported as an issue and 'fixed' previously. I only first encountered it this week.

Additional Notes:

1) I am currently only seeing this issue with one trade partner.
- This partner is on another continent and initially their lands were mostly hidden when I started trading. However, this is true for other cultures which did not have issues.
2) I have other resources which are currently trading normally with this partner.
- I think the currently affected resources are Ebony, Gold, Gemstones, Porcelain and Papyrus
- I have other resources, both luxury and strategic which are not breaking yet.
3) I suspect but am not sure that it started with one or more specific resources and spread to more resources showing this behavior.
4) I have tried re-trading the resource (losing up front costs each time) over multiple turns to see if it would go away. It did not. These different attempts were also made as we changed our trade status to be more open over time and as I discovered technologies that changed the resource purchase costs.

Somewhat related but I'm a little put off that these trade maintenance fees seem to make the money resources questionable investments without a significant reduction from pro-trade agreements and technologies. It's easier to gloss it over for other resources but I would hope that money-generating resources should always be a net gain. When these trade routes do get canceled I tend to be making more per turn without them.

Bumping this topic as I can’t seem to find any fixes or explanations about this. I’ve been having the same issue, with multiple trading partners.

It also happens to any resource not just specific ones. Sometimes, I would buy one thing (coal) which would be fine. Then when I proceed to buy something else (salt) the coal trade would disappear, but the salt stays. Do the same thing to the first resource and the second would disappear! Makes me feel like I’ve hit an upper limit on amount of resources traded or something.

Edit: I feel like it seems to happen more with Empires who own the most types of resources out of all the AI empires (i.e. the leading trade partner). In my game it first happened with Yellow, who then lost a whole lot of territories to rebels. Orange became the one with most resources next (with whom I’ve been trading longer and had already bought all their resources in trade). Now I’m starting to lose resources from Orange but the few remaining trades with Yellow are intact.