Can only PWM 2 Noctua fans at a time on a SM X9DRi-LN4F+ (but they run fine at full speed)

[wemcdonald]

TLDR: If I plug in 3 Noctua industrial fans for PWM, none will spin up. If I plug in any two of them (or 2 + a rando fan), all will will work fine.

I recently got a (new to me) X9DRi-LN4F+ SuperMicro motherboard inside SC846E16 4U chassis.

After researching the best way to quiet it, I replaced the stock fan wall with 3x 120mm Noctua NF-F12 iPPC-3000 fans. I tested one out, and it worked fine. I then built the new fan wall, plugged everything in, and... nothing.

This is what I've tried:

• If any 2 of the fans are connected to the motherboard (in any of FAN A/B or 1-4 headers), they'll spin up fine (nice and quiet)
• If I connect a 3rd fan to the backplane (which doesn't provide PWM) or use a 4->3 pin adapter cable connected to a header pin on the motherboard, the third fan will spin up at full speed
• If I connect all 3 directly to mobo headers, none of them will spin up – they get a tiny jolt of movement when power goes on, but that's it.
• If I plug in 2 of the Noctuas and a random 120mm Arctic F12 PWM fan I had lying around, all three will spin up and work fine.

Other things to note:

1. I've reset IPMI to factory defaults
2. I replaced the 2 rear fans with 80mm Noctua NF-A8 PWMs connected to FAN5/6 and they PWM fine.
3. This problem persists under all fan profiles
4. I've adjusted the IPMI thresholds for all the Noctuas to be in line with their published speed ranges

I could (reluctantly) live with 2 Noctuas + another fan, but it's driving me crazy that I don't understand what the problem is.

Any ideas?