Yes, the bug was damage was tied to charge rate, but not RoF related as you said.
https://www.reddit.com/r/DarkTide/comments/17qx6tv/plasma_gun_charge_rate_and_damage/
As of Traitor Curse P2 Patch this behaviour should no longer exist.
TLDR: Plasma Gun ‘normal’ (left click) shots do different damage based on the Charge Rate of the gun. The ideal CR is 73, but 64 and 80 are also peaks of sorts. Slightly more CR (+1-4) than those numbers is noticeably worse but slightly less (- 1-3) is only slightly worse (see table).
At the absolute worst (65 or 48 CR) you’ll do 5,3% less damage than a 73 CR otherwise equal gun. Into Flak armor that’s worth (in weapon stats) 17 Damage or 11 Stopping Power (or 7 Damage + 6 SP).
Here’s the table, going down to 47 Charge Rate. The CR and the percentage on the right are the numbers to pay attention to.