I had a similar issue on a website but fortunately it was a plugin I’d written so it was easy to identify the problem and fix it.
Have you identified which of those plugins is causing this? Or is it the theme at fault? Temporarily deactivate plugins one by one until you find the culprit. Switch to a different theme to see if that changes anything.
Once you know, ask this question in their specific support forum. Maybe that plugin/theme has been or needs to be upgraded since the core WordPress upgrade.
I would be tempted to restore the sites to your last good backup until you find a fix on one site – but I realise that’s a drag when 12 sites are involved.