Why does Twitter keep opening in a container?

I've just started using containers. However, Twitter keeps opening in one container (which I've named 'Baby') and will not open in the others. But I have not assigned it to any particular container. I have checked the lists of sites assigned to each container and it doesn't appear anywhere. If I try to open it in the default tab - or in another container tab - it immediately opens in the one particular container. Any advice please?