Boot Camp only detects two out of three monitors

Setup: brand new MacBook Pro with Retina Display plus two 27" Thunderbolt displays. So, three displays in total, which works like a charm in OS X.

Unfortunately, when boot-camping into Windows 8.1, it detects only two of these: the one on the MacBook and the (first) external one:

Note that interestingly, the devices plugged into the secondary external monitor still work - e.g. all three cameras are there.

Any ideas?