Although not an “answer”, I can give you my results on a slightly newer version running on AIX.
Sorry that I’m unable to replicate your problem.
[lholtscl@ibm3 ~]$ python
Python 2.7.13 (default, Sep 7 2017, 21:08:50) [C] on aix7
Type "help", "copyright", "credits" or "license" for more information.
>>> print "%.10f" % 123456789012
123456789012.0000000000
>>> '{0:.10f}'.format(123456789012)
'123456789012.0000000000'