LiveView falls back to longpoll after dev server restart

I’ve noticed that when I kill and restart the Phoenix dev server, all existing browser tabs with a LiveView reconnect back to the server using longpoll (and stick to using longpoll). If I open a new tab, the LiveView connects using websocket.

I can’t say I’ve noticed this before, and I’m not sure if this is expected behaviour or something I’ve tweaked and set up wrong?