Gauss' law is a physical fact. The field line is a metaphorical construct to help us visualize the electric field distribution.
An unspoken implication in this metaphorical construction is the expectation that one, and only one, field line goes through a given space point. It is this expectation of the metaphorical field lines that is violated in the cited situation.
At a given x=0 space point, several field lines coexist. The usual computerized field line construction algorithm will balk in this situation.