why the buff to dia again again?

There's definitely a messy history here. Our aim throughout has been to provide a more consistent experience, but there have been other bugs exposed as we applied various fixes in pursuit of that goal, which required new fixes, and so forth. Fingers crossed, but we think we're at a final state at this point.

The core of the original problem lay in Dia's nature as a Shadow-school-exclusive caster with a variety of spells, one of which (Void Bolt) needed to respect school lockout when interrupted, and Mark of the Necromancer's exponential growth that makes Reap timing very important.

A history of changes here:

Original state: Nightmare Visage respected school lockout, and because Reap could not be cast during Nightmare Visage, the exact timing and quantity of Void Bolt interrupts could cause huge variance in Reap's timing, and thus the damage taken from Mark of the Necromancer. Some players figured out an exact interrupt order to reliably cause Reap to be delayed so that it started coming after Mark, pretty much trivializing Mark for the rest of the fight.

Reap Hotfix #1 (August 3): Because it was a problem that the danger posed by Mark/Reap could vary drastically from pull to pull, we made Visage ignore school lockout, which made its timing more predictable, and thus Reap's timing as well. We realized that many guilds had only killed the fight by manipulating the timing to get the optimal Reap/Mark order, so we also nerfed Mark damage at the same time to help offset the potential increase in difficulty.

Post-Hotfix #1: This mostly addressed the problem, though quirks in AI timing meant that it was still rarely possible to get the "easy" spell order. The new timing exposed an even worse bug, however. Dia could not Reap while Darkness was occurring. Previously, delayed Visage casts tended to cause these two abilities to not intersect. But after our hotfix, there was a predictable window, about 6:30 into the fight, where they would line up, and the result was to delay Dia's fifth Reap by a significant amount, causing Mark damage to ramp out of control and almost certainly cause a wipe. Not good.

Hotfix #2 (August 11): We fixed the issue with Darkness preventing Reap, and rebuilt the way Reap/Mark work, and now force Dia to re-cast Mark after each Reap so that they can never get desynced and the variance in Mark damage from pull to pull should go way down.

Post-Hotfix #2: The change to Darkness/Reap interaction also increased the effective duration of Darkness (a side-effect, not the core intended change), which increased the damage and positioning disruption from that ability.

Hotfix #3 (August 13): We're going to be reducing the duration of Darkness to match how it used to be prior to Hotfix #2.

We're generally hesitant to buff fights once they're in the wild, and didn't set out here with the intent of making the encounter strictly more difficult (hence the offsetting nerf to Mark damage in Hotfix #1 above). The goal was always to reduce large variation in the fight's difficulty from pull to pull, which makes for a frustrating experience. There were admittedly a few more pitfalls along the way than we expected. Please let us know if you continue to experience any frustrations or apparent bugs here.