@burnout426 @leocg I wasn't using multiple windows, but I discovered now that my "On startup" setting got reset again to "Begin fresh with start page" like it happened in the last autoupdate.
This time, though, I'd done something that must be related to that (I didn't mention it before becaus it didn't occur to me that this setting could be the culprit so I just didn't add two and two): I re-enabled synchronization in my Opera Developer installation, which I have set to always open the same set of tabs.
To be clear, this is a bug. I'd unchecked "Open tabs" in the sync settings so that my tabs in both browsers would be separate from each other, but I kept "Settings" checked by mistake. None of the two browsers had that setting enabled for startup, though. Opera Developer had "Open a specific page or set of pages" and Opera Standard had "Retain tabs from previous session". None of them had "Begin fresh with start page" so this shouldn't have happened.