I have things like CanvasBlocker, uBlock Origin, Tree Style Tabs, etc, and disabled third-party cookies. But nothing that should actively interfere with it. wrt containers, I have Multi-Account Containers, Containerise, and Temporary Containers. It happens every time I go to a URL that has an associated container I've made (YouTube, Twitter, Facebook etc).
I might try resetting my container plugins to see if I can clear it.
Fixed it. Containerise has a separate "Keep old tabs" setting that is different to the Multi-Account Containers "Replace tab" functionality. It works as intended now. Horray!
