By way of an example, I went to the Macrumors iOS15 page and used “open in background” (cmd-click on the magic keyboard) on the first three threads listed there. Two of them loaded as expected, the third gave me the blank start page. If you look closely, the correct URL is shown in the location bar, but the tab title says Start Page. Reloading the page via the reload button, cmd-R or option-cmd-R does nothing whatsoever. The only way forward is to close that tab and try again.
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Edit to update: when I click the URL in the location bar of the dodgy tab to highlight it and then press enter, the page loads. It’s really jarring and drives me nuts.