This is unfortunately a known issue. Since your material is only used during mouse over, it might happen that it isn't attached to any object, it might not get restored when the context itself is recreated. This is already solved for the next Flare3D version, but for the time being, you can re-upload the material by hand whenever you get a CONTEXT3D_CREATE event. Just call:
material.upload( scene );
And your problem should be solved. Please note that there's no need to actually recreate the materials themselves since those are preserved by Flare3D no matter what, you just need to reupload the problematic ones by hand to the GPU.