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This is meant to make the PFC appear roughly as a slowly varying resistance from the perspective of the grid (a positive resistance, specifically).

When the direction of power reverses, it will make the PFC appear as a slowly varying negative resistance. This can still result in stable operation if the grid's impedance is lower than the magnitude of the PFC source impedance (see the middlebrook criterion). Seeing unstable operation at higher grid impedance is the expected behavior from your control law.

So you shouldn't use the same control law when operating as power source to the grid. I'd suggest adopting the control strategies used by grid tie inverters. I'm not an expert on this, but I'd be willing to bet that from a regulatory perspective this will be necessary as well (even if your system were stable, raising the grid voltage significantly is likely not allowed).