Got an internal update for this issue:
This behavior of the shells is present for a long time and occurs when you quickly move the aiming reticle right before the guns fire. It got more attention because of the "railgun" bug: it caused the change in behavior of the shells when reticle is moved before the shot. To fix the "railgun" we rolled the game logic back to the state before the changes which resulted in it appearing as a side effect. This situation prompted us us to have a more detailed look at older mechanics and after fixing the "railgun" we planned an improvement for the future.
However, developing of the proper solution for this "under-the-hood" firing behavior might take significant time because it is related to technically complex game logic and the way client - server exchange the information. We will obviously share more information when we will have a stable improvement ready.
Hope that helps!