That's a not a bug--it's a feature!
The diplomacy bug I've found in 1.52 happened as follows:
1) I asked for dyes and Mansa Musa was happy to help me.
2) When the time came, he ended the gift and so I asked him what he wanted and nothing changed. That is, I could have dyes again for free.
3) So, I left diplomacy and came back and asked for dyes again and he said "I pressed him to hard" and refused.
4) So, again I asked for what he wanted, and he was willing to give dyes for free.
I have a save, if anyone wants to try it.
This never really cleared up, either on subsequent turns.
Edit: I just tested the save, and when I accept the "free" dyes (as in point 4 above) it turns out it is actually a trade of dyes for silk. I just don't see the silk half of the deal on the trade table.