Holly problem

I think it may be trying to tell you that it wants to spread its roots. I had exactly the same thing happen to my Camelia in a large container. It is now happily growing in the ground.

Ilex generally and particularly the meserveae types if planted into too large a container for their rootball can show these symptoms. Because their roots grow slowly, too large a body of surrounding soil/compost stays too wet and they start to drop their leaves prematurely. I have come across this several times over the years.

Your only option is to plant into a small container slightly larger than the rootball. Put a Viburnum tinus 'Gwenllian' in it's place instead.

Well I've learned something there Fractal (one of the benefits of GoY).
I just assumed that it must be the same problem as my Camelia suffered. I think the same applies to those lovely blue African Lillies, forgotten their proper name. They like their roots to be confined don't they?
Sorry Duck, got carried away!