errorLevel E_ALL breaks debug kit?

Hii there,

I’m trying to work my way towards upgrading my CakePHP 3.9 app towards CakePHP 4.
For this, I, of course, need to fix a few dozen deprecations, but here is the problem… I don’t know what deprecations because the debug kit doesn’t show me.
When I have the errorLevel set to E_ALL & ~E_USER_DEPRECATED (which ignores all E_USER_DEPRECATED messages), my debug kit shows the following:

But at least my app works besides this…
Now when I try setting it to just E_ALL, instead, my debug bar gets hidden, my app starts behaving weirdly and when I look at the source (crtl + u in Firefox), I see that it just slams the warning into there (as opposed to getting intercepted and put into the debug kit).
Is there anyone here that has a similar issue and knows how to fix it?

EDIT: I fixed two deprecations in my bootstrap.php, now the debug kit tells me I have 12 deprecations but when I click the panel, it just shows me the error in the screenshot.