Recently there was a thing where VAC would erroneously flag AMD’s antilag+ feature as cheating, and issue a ban.
AMD then quickly disabled the feature by default but now Valve also patched detection for it and is now, at least according to these patch notes, reversing the bans.
They were legitimately picking up an activity that they generally try to prevent. AMD was using a technique to apply Anti-Lag+ that was expressly monitored for, to provide anti-cheat.