Tom, thanks for writing back.  Wow, that’s great!  You seem to be in the minority though: Since posting, I found there’s an Issue on GitHub 9 where people are reporting the behavior I describe.
So, what is it that you’re doing that’s different from the rest of us?
One thing seems to be that you’re running for more steps (25): the example stops at 15.  But I don’t think that would make a difference, because the loss seems to “flatten out” fairly early, e.g. by step 3.  Output is…
STEP:  0
loss: 0.537086230955
loss: 0.519054139134
loss: 0.334312643402
loss: 0.245408088178
loss: 0.241630530524
loss: 0.236310649411
loss: 0.227821229553
loss: 0.212925355116
loss: 0.18457783161
loss: 0.134251495296
loss: 0.0847459995036
loss: 0.0574250318391
loss: 0.0512263487439
loss: 0.0473003375388
loss: 0.039150461162
loss: 0.0352442710805
loss: 0.0323013329392
loss: 0.0247076271594
loss: 0.0190413202173
loss: 0.0149685327246
test loss: 0.0122880477308
STEP:  1
loss: 0.0123111050907
loss: 0.00789734904096
loss: 0.00671599497278
loss: 0.00353805780334
loss: 0.0032202868875
loss: 0.00275785860848
loss: 0.00255092406484
loss: 0.00235055785575
loss: 0.00218358551007
loss: 0.00186726224563
loss: 0.00158670291558
loss: 0.00253018196455
loss: 0.00124947355056
loss: 0.00129163893022
loss: 0.00115403097926
loss: 0.00113527961892
loss: 0.00111034928823
loss: 0.00110101619577
loss: 0.0010856783608
loss: 0.00107512001811
test loss: 0.00151556788016
STEP:  2
loss: 0.00106017296511
loss: 0.00101216329981
loss: 0.195079458435
loss: 1.49997814199
test loss: 1.50362019943
STEP:  3
loss: 1.49997814199
test loss: 1.50362019943
STEP:  4
loss: 1.49997814199
test loss: 1.50362019943
STEP:  5
loss: 1.49997814199
test loss: 1.50362019943
STEP:  6
loss: 1.49997814199
test loss: 1.50362019943
STEP:  7
loss: 1.49997814199
test loss: 1.50362019943
STEP:  8
loss: 1.49997814199
test loss: 1.50362019943
STEP:  9
loss: 1.49997814199
test loss: 1.50362019943
STEP:  10
loss: 1.49997814199
test loss: 1.50362019943
STEP:  11
loss: 1.49997814199
test loss: 1.50362019943
STEP:  12
loss: 1.49997814199
test loss: 1.50362019943
STEP:  13
loss: 1.49997814199
test loss: 1.50362019943
STEP:  14
loss: 1.49997814199
test loss: 1.50362019943
…and all the predict*.pdf files from predict2.pdf onward look like the graph I posted.