The overset implementation (and presumably the ALE implementation as far as I can tell?) does not properly account for the motion of the background mesh when calculating symmetry boundary conditions. As a result, the normal velocity on these boundaries of the moving mesh will set the normal velocity equal to zero, instead of the normal component of the mesh velocity. The proper way to handle this seems to be to pass the mesh velocity into the symbc() function, in a similar way to how the it is passed into, e.g., CompFlow::rhs().