I do not have this problem in general, but I do have one repeatable case. I have roboform password manager and it is supposed to work with Opera, but the regular plugin will disappear, it will work perfect when it's there, but again, I don't think it ever stay for more than a few hours. There is no trace of it in extension, I have to restore it. The lite version, appear to work fine.
I have a laptop and both desktop and laptop are running (as near as I can control) the same software (no clearing) but the regular Roboform extension works just fine (no disappearing act) on the laptop. Something is different, just don't know what. I think the question really is what will cause an working extension to be totally removed?