This past week, Blizzard created a new support article acknowledging that the Theramore Tabard and Mini Mana Bomb were not being properly awarded after completing Theramore's Fall (A), though intrepid players have discovered the key lies in completing the scenario at exactly level 35 in order to earn the rewards!
The original pre-patch scenario rewards are now available to all players in MoP Remix.
Level 35 seems oddly specific, but as explained by Warcraft Wiki, the original pre-expansion scenario required level 85 characters, and although the scenario could also be completed at level 90, it no longer offered the tabard or toy following the full release of Mists of Pandaria. Post-Shadowlands level squish, level 85 correlates to level 35 today, and through some odd fluke it appears that the rewards are only available to characters at the same level they would have been during the Mists of Pandaria pre-patch.
Courtesy of Redditor u/Doomhammer24.
This curious level requirement is almost certainly a bug that will be addressed at some point, but leveling new characters is easy and even one of the simplest ways of farming bronze now, so spin up a new character and complete the scenario to make sure you get the rewards.