Most of the times (although not always) background tabs are loaded but not visible. I think this has to do with the content (css perhaps?) as some (the minority of) sites are not bitten. I could test this issue only in webengine, since opening background tabs does not seem to work when using khtml/webkit engines (perhaps I should fill another bug about that).
Oops, I just realized that this does have to do with whether the tab is opened at background or not. It happens regardless of that, and only in webengine. The way to reproduce it is to open a link with middle click (the issue is not triggered otherwise). To workaround the issue (make the page visible), I need to switch the engine, e.g., from webengine to webkit and then back to webengine again.
Couldn't reproduce this anymore with Konqueror 22.08.3, Frameworks 5.101.0 and Qt 5.15.7. Seems to be resolved.