Wearing a heroic rampant thornstrap belt - bonus roll gave me yet another heroic rampant thornstrap - the bug is back
You did not mention how you received the current equipped heroic rampant belt.
If this came from a chest, and you received a bonus roll heroic rampant belt, then the bonus roll is working as intended as that is a seperate roll on a loot list from the boss which you can preview before accepting a bonus roll.
Should you see bugged rolls or think it is bugged, please report it through the ingame bug reporter, thank you.
I had the same situation yesterday. I received the second [Custodial Cuffs] 1/6 Mythic from Magister’s Terrace +10 instead of [Gloves of Viscous Goo] 1/6 Mythic (the last one mythic item I’ve never received).