This is absolutely, certainly a bug, and a major one at that. Here are more screencaps to further demonstrate the issue.
Here you can see that requesting all of an AI's Gildar when they are at war does not cause them to massively overvalue it.
The issue also isn't only caused by asking for all of an AI's Gildar- here I have requested under half, but still am met with an insane demand. The problem also isn't that the AI has some plan requiring large amounts of Gildar, and therefore refuses to trade away its stockpile- I can complete this trade in four 50-Gildar installments. The next two pictures should make the bug as clear as I ever can:
The AI considers this trade eminently acceptable.
Delete the 100 Gildar figure, type in 110 instead, and the AI freaks out. This is a bug.
In addition to the above, like I noted earlier there is a second, related issue-
Once you have encountered the bug, the massive AI Gildar request remains on the table, preventing further deals until you exit the trade dialogue.
Please test it for yourselves if you still doubt that this is a problem. I am disappointed that FE has such a major flaw in a very basic aspect of diplomacy.