Fractal GP-12 not spinning at full speed.

[unknownmiscreant]

2 days ago I purchased a Fractal design GP-12 fan:

 

https://www.elive.co.nz/fractal-design-dynamic-series-gp-12-case-fan.php

 

Its a great fan, super silent. I really like it, however I have noticed that when placed steel objects, such as my PC case, the RPM drops to around 400, from 1200. This can be heard, felt (by using a finger to slow the fan down) and measured using a motherboard fan header. 

 

No cables, connectors, are faulty. I have already exhausted this line of troubleshooting. The fault occurs even when the fan is connected directly to the 12v and GND lines of the PSU. I have a custom fan splitter made up that passes PWM and fan speed to the motherboard, and powers the fans from a molex connector. I have fully tested this, it is NOT faulty, and has been working perfectly with some other fans for a month or so.

 

I have measured the voltage at the fan connector using a multimeter, and it is 12v.

 

Leaving the fan standing vertically on a non-steel surface, and bringing a steel sheet against the fan causes the issue (the fan remains perfectly still throughout.) So I think it is highly unlikely that the cables, connector or PSU are at fault. I have also verified the fan header I had been using to read fan RPM is working correctly, using the frequency counter on my multi meter.

 

The fan motor is magnetic (when powered and not powered,) causing it to be attracted to steel objects.

 

Has anybody else encountered this issue, or is my fan just plain faulty? I can't find anything on the internet about this.

Thanks in advance.