Style Sheet Flicker

Bit of a weird one. I am using a Kale Child Theme.

When the site loads, it seems to displays with the default CSS but then after a second, applies the custom css from the child theme. This creates a strange flicker as the style is updated.

Originally, I had the style in the custom css but then moved it to the child style sheet. This made no different. I disabled caching but again still happens.

Any pointers where to look for this one?