A/B test adjustment only visible on new page view

During the A/B tests that I start, I often see the following happening:

I open a page that is part of the A/B test and do NOT see the variation change
I refresh the page and see the change, so I was in the variant
So it seems that I don’t see the change in the variant immediately, but only after I refresh the page again. So I never immediately enter the variant when I come to the website.

It all works fine in the debug mode of Google Optimize.

I have the Activation Event set to ‘On page load’, but maybe I should work with ‘On custom event’?