I've run into this problem in the past, as well. Nobody's able to tell me what I did wrong, or what technically happened, but I did find a "work-around". I'm not at my IV station right now, so please bear with me. When you have the iPart parent .ipt open, expand the various sizes in the browser. When you "drill down" to the final part (each variation), you can RMB on it, and there's an option for "Create files", or something like that. It will re-create the individual .ipt for that child, and once again, allow you to place it in assemblies. This would have to be done for each child, or at least, each child that's not behaving. All of your constraints in existing assemblies should be okay.
Inventor Professional 2021
Vault Professional 2021