Due to damage immunity, some of the guns with high frequency(e.g. SMG, Tommy Gun and Minigun), have low DPS on a single target.

Note: This bug does NOT always happen. Causes unknown.

From what I have noticed, it seems that if you aim slightly above the mobs head, the dps returns. Its even consistent across modded mobs, its a very small spot that makes it impractical to use, so i doubt its a "headshot" of sorts. For more information, i'd like to add this is happening to me in 1.20.1.