There is a "treaty" associated to accepting gifts, that incurs Reliability drop when declaring war soon after (10 turns, as usual), probably to simulate 'paying tribute'. It probably merits "Informal Expectation" treatment, even if diplomatic bonus decays longer than that.

These diplomatic relationships are somewhat rare in this game, and I didn't have enough 'feel' for them to tweak appropriately. Will have to revisit these later at some point.