This is exactly the cause for the FAN RPM's fluctuating. Many many many fan extension cables out there connect all the FAN RPM signals (All the ones I have all do). Quick fix is to disconnect the RPM pin from all the fan extension sockets except from the last one in the chain before the wires go into the Fan header.
This isn't unique to the Aquaero 6 XT either, it's an issue anywhere you have multiple FAM RPM signals going to the same Fan Header. Ended up in an argument with a Youtuber who I saw this issue on his Fan Controller in a video he posted and when I pointed it out he completely lost his **** and said he was an 'expert'.
LOL! Yep, this has sorted it!