Swiss Meringue Buttercream help!

Hi all, I'd love some help. In the last few weeks I've made SMB twice and the meringue whips up perfectly glossy and stiff, once the butter is added it gets to the silky smooth and shiny stage no problem. However, when I apply my final coat of buttercream to the cake, it seems to split and go grainy. I'm really not sure what is happening and how to prevent it! Any advice would be appreciated.

I've added some photos to show what I mean.