Did you press F after equipping the M79? Apparently, the buckshot is an alternative fire-mode.

Any progress about this issue?

The video is self-explanatory.

After doing more investigation, the engine sound quality of the Zamak weakens as my third person camera moves away from the vehicle (in third person). But when I force my camera to come close to the Zamak's front side, the sound quality returns to normal.

As of right now, I think the developers are setting the Arma 3 Contact expansion as their highest priority. Since that expansion will be released in July 25th, they are spending more time on Arma 3 than DayZ; the developers have been publishing partials contents to the DLC dev branch thus far. Once they are done with converting the beta-content to the final product, they will most likely get their attention towards DayZ.

The developers are already aware of this issue; it should be fixed now.