The B-90's analysis contains the following part :
"Like all behemoth class robots, the B-90's overall structure is slightly weak against kinetic attacks,"
However, Behemoths are generally slightly weak¹ to thermal damage, not kinetic damage.
¹They're either neutral to or weak to thermal damage, even with their reflective armor.
They are never weak to kinetic damage, only ever neutral to it.
Edit : Found the source!
* MOD: Behemoth core switched from kinetic weakness to thermal weakness
From the changelog for Alpha 12.
The date in the changelog section for that version is missing an end parenthesis in changelog.txt, by the way. It currently looks like this :
Cogmind Alpha 12 "The Place No One Lives to Tell About" (161122