I've ran some tests and tenacity does seem to be working as intended. This was the result in a full set of Dire against the 2nd boss.
Hard Mode
| Tenacity | Min Damage | Max Damage |
| 22 | 11 | 20 |
| 0 | 31 | 40 |
Normal Mode
| Tenacity | Min Damage | Max Damage |
| 22 | 9 | 16 |
| 0 | 24 | 31 |
If possible, could you provide details on your playthrough and anything you might have done to your stats. Just to rule out the possibility that you encountered an obscure bug that could have potentially led to tenacity not working.