If OBJ models created with a previous version of AOI are still correctly imported, it means that AOI changed something (it looks like a possible regression since version 2.9 brought changes on textures and materials). You can change it the way you did, but you can also change the value following "d" in the MTL file, 0 being fully transparent and 1 fully opaque.
If you're sure it's a bug, it would be nice to report it to AOI developer.