The ticket #24072 has been positively reviewed, meaning that it will be included into the next SageMath release. The solution found by developers is to forbid to mix elements from the symbolic ring SR and elements from finite fields. The reason is that the symbolic ring is inherently a ring of characteristic $0$. Your example will not result in a segmentation fault anymore, but you'll get an exception TypeError: positive characteristic not allowed in symbolic computations.
In your case, the solution is to work with either a multivariate polynomial ring (P.<x, a> = GF(3)[]), or you may want to have rational functions in a for instance, using P.<x> = GF(3)[] and F.<a> = FractionField(P['a']).