I guess not a bug asmeurer because if are dealing with expressions with only monomial t that is something like the second test case and call special function in prde.py, indeed we turn up passing constant terms in the reco function as we tend to evaluate b/a at sqrt(-1) and pass only the real part of the evaluated expression [hence a definite None]
For the rest of the cases that is when i have t in my polynomial i tend to play with the domains trying to unifying ZZ with ZZ[x,_z] which is a problem in itself