I just did a fresh install of Wow Classic on a spare computer and installed only the Dugi TBC addon. I tested a toon that’s been sitting idle in Loch Modan and selected the Midsummer Events guide. When I clicked on the fire in Loch Modan, the addon marked Elwynn Forest as complete.
I tried the same thing on a horde toon in Razor City. Upon clicking the fire the guide marked Mulgore as complete. If I select (from within the Current Guide window) Reset and Reload the correct quest is marked.
This does not appear to be a horde vs alliance issue, nor the specific Windows/WoW installation or addon compatibility.