I too have experienced this glitch. Here are my findings.
Never observed Berserk malfunctioning by itself, only when I have Killing Frenzy also:
Killing Frenzy and Berserk seem to work only intermittently together. When I first got the two together (on a Billhook exclusive user), it would only proc Killing Frenzy the first kill he got in combat, and never both, but it worked synchronously after that, extending Killing Frenzy and procing Berserk together: This happened reliably for four battles, all against Orcs, so I thought it was a feature, not a bug (which saddened me considerably).
Then I used this same fellow against bandits and the undead, and I observed Berserk AND Killing Frenzy procing against the undead every time (that I observed), and about 80% of the time versus bandits.
The conflict seems to be the first time they both try to proc together, although the battles rarely lasts long enough to afford more than two or three opportunities to use the skills. My final party had two brothers with identical builds that incorporated both Berserk and Killing Frenzy, and I can’t say with 100% certainty, since the second fellow’s faced fewer Orcs, but the latter of them used 1H and shield, and he seemed to have no trouble procing both Berserk and Killing Frenzy the first time in combat. He’s seen fewer combats, however.
Really glad it seems the two are supposed to go together :)
My suspicion is that Killing Frenzy and Berserk may not work perfectly with 2 tile ranged weapons. Not a whole lot of testing to back that up, but I first noticed the problem (in repetition) with a Billhook user. As soon as I figure out where to find my savegames, I may get into a battle that demonstrates this and attach it; been binge-playing your game.