It might be caused by your balloon collision being too thin. The FLIP solver can't properly handle collisions that are too thin. Make sure that your collisions are around two flip-sized voxels thick and that the division size of collision is at least the same size or lower. If the problem still persists, I can try and help further.
Hope that helps,
Okay I see. I believe GT is meant to be the preconditioner matrix. You should be able to pass in 0 for GT's pointer and the solver will default to diagonal jacobi for the preconditioner.
Surface tension is handled as a field, so there is no particle-based attribute. It essentially computes the curvature of the fluid surface at each voxel and scales that value by the global surface tension coefficient in the FLIP solver.
It's possible to achieve variable surface tension by creating your own attribute, stamping it to a new field (using Gas Particle to Field) and scaling the surfacepressure field by this new field.
Hope this helps!