The Dual Nanopulse Mek'leths have had their animations changed, it seems. Before when holding or mashing the R2 button/trigger for the primary attack ("Quickening Strike"), it would start the next part in the chain once the current attack finished its animation, as per normal with melee weapons. Now they bypass the animation and start the next one super quickly. This makes for a lot of super fast strikes and a lot of damage that I'm sure is unintended.
In the video below, it first shows that the secondary attack ("Overpowering Strike") is working fine and letting the animations finish before the next swing starts. Then when using the primary attack ("Quickening Strike"), things get weird.
https://youtu.be/ehdUS6vXMa0