I have the exact same issue since the latest update. I have 4 LL fans, 4 RGB strips, K95 platinum speed, Glaive and a Hydro H115i (old model) installed. All parts are working fine with the exception of all 4 fans. They start flickering in absolute seizure mode after a fresh (cold) boot or after waking out of sleep mode. Exactly like above shown GIF, even though rainbow is not my default color setting.
Everything was working fine prior to the update.
Only solution so far is to completely reboot from Windows. So not really a proper way to fix it.
Small side note: I did swap RAM modules that same day but I can hardly believe that has anything to do with it.