that is a serious drop in RPM. The stock fan would be doing 4000RPM at those temps.
possibly a rporting issue. the fan is capable of 2000rpm and the antminer software would certainly be feeding it full amperage at 60C temps. it might report 1200 but likely is 1600+
I thought it was a misread too but the speeds of the fan DOES increase then decrease and the software does report the changes. I'm not sure why it's performing like this, the noctua fans are designed to push air more streamline. I thought that could be it. Whatever it is it's working and my house is quiet. With another fan speeds were in the 3000's and loud at this setting and temps were 5 degrees higher. It didn't make sense until I put my hand under the miner and with that setup there was a lot of air escaping from under the miner and not making it back to the heatsink. With the noctua fan there is no air escaping from the bottom side of the miner (my miner sits on its side) and if I put an accessory fan to pull or push from the bottom side temps go up and the noctua spins faster.
The 1200 rpm fan is capped at 1500 rpms, the noctua fan is at 1680 rpms.