Google ad not showing instantly when viewing postings

[Black Tiger]

I've created a code for my sidebare to show the ads. It's in fact the same code as I would put in the default advertisement, only I put it in a block to get a nice header above it.

This ad is shown instantly on Home, Forums, Downloads, shortly said all main pages.

When I open a forum it also shows instantly on the thread list, So far so good.

But when clicking on a post when viewing a post, this happens:



So I refresh my page and this happens:



Only after refreshing my page for a 3rd time, the ad appears.
In a very few cases after 2 times refreshing the browser.

This is happening with newest version of Firefox, Chrome and Edge.
Only difference is that with Chrome, sometimes you only need to refresh once.

This is *only* happening when viewing posts.

Nowhere else, not even on self create pages. Only on post viewing.
At the same moment that this is happening, also the footer ad is not displaying anymore. So imho it can't be caused by my custom block.

What's happening here?

Lazy loading is disabled, Ajax is disabled.
PHP 7.2.15
Apache 2.4.38
Mysql 5.7.24