With 7.6beta3:

sage: f.numerator()
a01*a10^3*u00^2*u01^2 - a00*a10^2*a11*u00^2*u01^2 - 
a00*a01*a10^2*u00*u01^2*u10 + a00^2*a10*a11*u00*u01^2*u10 + 
a01*a10^2*a11*u00*u01^2*u10 - a00*a10*a11^2*u00*u01^2*u10 - 
a01^2*a10^2*u01^2*u10^2 + a00*a01*a10*a11*u01^2*u10^2 - 
a00*a01*a10^2*u00^2*u01*u11 + a00^2*a10*a11*u00^2*u01*u11 + 
a01*a10^2*a11*u00^2*u01*u11 - a00*a10*a11^2*u00^2*u01*u11 + 
a00^2*a01*a10*u00*u01*u10*u11 - a00^3*a11*u00*u01*u10*u11 - 
2*a00*a01*a10*a11*u00*u01*u10*u11 + 2*a00^2*a11^2*u00*u01*u10*u11 + 
a01*a10*a11^2*u00*u01*u10*u11 - a00*a11^3*u00*u01*u10*u11 + 
a00*a01^2*a10*u01*u10^2*u11 - a00^2*a01*a11*u01*u10^2*u11 - 
a01^2*a10*a11*u01*u10^2*u11 + a00*a01*a11^2*u01*u10^2*u11 - 
a01^2*a10^2*u00^2*u11^2 + a00*a01*a10*a11*u00^2*u11^2 + 
a00*a01^2*a10*u00*u10*u11^2 - a00^2*a01*a11*u00*u10*u11^2 - 
a01^2*a10*a11*u00*u10*u11^2 + a00*a01*a11^2*u00*u10*u11^2 + 
a01^3*a10*u10^2*u11^2 - a00*a01^2*a11*u10^2*u11^2

In general you're advised to use polynomial rings when dealing with 
polynomial fractions.
As to the bug, Pynac switched to Singular for GCD and some interface bugs 
turned up around Sage-7.5

Regards,

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