I can appreciate the sinking feeling this must have given you, but it may not be much of an issue. If you want to be extra careful, those stones over the pipe could be removed and then replacement stoves with no backing put in place. That said, it seems like there would be little chance of a problem. The plastic in the mesh is just a coating. It is encapsulated in mortar and completely surrounded by non-combustibles so the likelihood of this becoming a problem seems very low. That's my 2 cents, though I am not a mason so this is just an opinion. I'm sure others will chime in with theirs.