Brief bug summary:
Jett's skills that give mastery (Gun Mastery and Giga Blaster) only apply client-side, but not server-side.
After recording this video, I had somebody tell me how much damage they saw when I attacked. Some of my lines were actually only 30m when I saw 180m. This leads me to believe that Nexon only fixed the Jett mastery glitch on the client-side. However, server-side damage calculations still do not use mastery from "Gun Mastery" and "Giga Blaster".
TLDR: Jetts don't do the same damage they see on their screen, and the mastery glitch was never actually fixed.
Each character has a damage range, MinDamage - MaxDamage, which is dependent on their stats and equipment. When the game needs to calculate the damage they do, it randomly generates a number between the MinDamage and MaxDamage, and then multiplies that by the skill's %, the various buffs the char has on, and damage reductions the enemy has.
"Mastery" is the stat that controls how wide the gap is between MinDamage and MaxDamage. It is the ratio between them. At a theoretical 100% mastery, every hit of the same skill on the same enemy would do the exact same damage. At 0% mastery, any hit could be any number between 1 and MaxDamage (times skill %, etc).
Obviously, then, a character with no mastery will do half the average damage of the same character with high mastery.
Displayed damage numbers
When a character uses an attack skill, the client immediately calculates how much damage this attack will do to the enemy, displays
those numbers on the screen, and sends them to the server.
The server, in order to prevent damage hacks, ignores
these numbers, and calculates new ones. These server-generated damage numbers are applied to the enemies and sent out to any other users on the map, to be displayed on their screens.
So, if two people are on the same map, they each see slightly different damage numbers from each other.
Normally this doesn't make much of a practical difference, because all classes have good enough mastery that the difference between damage numbers generated by the client and the server is negligible.
The v.183 patch on March 1st broke Jett's mastery skills (Gun Mastery and Giga Blaster), effectively halving Jett's damage by dropping the bottom out of her damage range.
Minor patch v.183.4, on March 9th, supposedly fixed these skills.
And indeed, after that minor patch, the range displayed was narrow again, and damage numbers displayed on Jett's screen were consistent as they should be.
However, as demonstrated by the video in the linked reddit thread, the damage actually done by Jett is far lower than what is shown to the user. As explained above, the damage that is actually applied to the enemies is calculated by the server, and is not the same as the damage calculated by the client and shown to the user.
Other users on the same map are able to see the damage numbers calculated by the server, and they report that the lowest ones are actually far lower than possible if mastery is correctly applied.
This leads to the conclusion that the v183.4 fix to Jett's mastery was only applied client-side, and the server is still doing its calculations without any mastery.
Steps to reproduce:
1. Be a 4th job Jett with maxed Gun Mastery (2nd job) and Giga Blaster (4th job), and with a gun equipped.
2. Check your damage range (in 'stats' window). Make note of the mastery applied (MinDamage / MaxDamage). It should be 85%.
3. Have another character on the map, observing you
4. Attack enemies
5. Compare the damage numbers shown on your screen to the damage numbers shown on the other person's screen.
In particular, observe that all your numbers fall within 15% of each other, but the other person sees some numbers that are considerably lower.
6. Conclude that mastery is not applied server-side.