Additional: Miximized windows on the smaller secondary screen appear correctly.
I can confirm, I have a similar problem on Windows 10. On my primary screen, on laptop, everything works fine. But on the secondary, external screen, when the Opera window is maximized, I only see the bottom right corner of messenger and there is no possibility to move it. The only way it works properly on the secondary screen is in the pinned state. But I rarely use this mode.
I haven't found any other thread about this topic, neither here in forum, nor in Google. Does anyone else has this problem? I switched to Opera mainly because of this feature, so it's a pity, when it doesn't really work correctly.