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Under the hood, the compiler is recognizing that the
function bodies are identical and it produces just one code
object instead of two.  Hence, the co_firstline number with
always reflect the first time the code is defined.

A simple demo shows the id being the same when the bodies
are identical:

src = """
f1 = lambda x=1: x
f2 = lambda x=2: x
"""

from dis import dis
c = compile(src, 'example', 'exec')
dis(c)

For the OP's code, the effect can be shown by
differentiating either lambda body by adding zero to the pop
index:
   nextkey1 = lambda N=keynums1,K=key_src: K[N.pop()+0]

I conclude that there is less to the bug than meets the eye
and that it may even be considered a feature.  Referring to
Jeremy for final disposition.