Unwanted, regular accents using Icon Bass

When using the Melody variant of any given preset or custom sound on pre-written MIDI, choosing the 'plectrum' option gives me an audible accent on every third note. It's a fairly pronounced transient throughout the mid-range, for the sake of argument 600Hz and up.
It's more pronounced on crunch or high-gain sounds, and of course on brighter ones.

This happens whether I write melodic lines or singe-note repetitive ones, and it seems to happen if the velocity is higher than 97-98.
It's as if though there's a pattern for a real or emulated pick attack, but without any sort of round-robin functionality - it rings out strictly every three notes.
Humanizing the line with various random seeds doesn't change this.

Is there some setting or a convention in how the instrument responds to MIDI that I'm missing? I can't find anything specific in the manual.
Any help would be appreciated.