I use a laptop and my main is the integrated laptop monitor, where the two externals are plugged into the GPU ports. I don't think that's a factor. Are you sure your main is really the one you think it is? The browser window will usually open on the monitor where it was previously opened - not necessarily the main monitor.
But the message box should by default pop up on the main one. It might also be that it pops up on monitor number 1, too.
If possible, maybe try switching up the ports, so their order switches and then see if the issue persists.
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