I use two identical monitors at work. Each has it's own panel and widgets. After a reboot they are being set to cloned output - same image on both monitors, and every time I need to restore my configuration manually, move around the panels and widgets to get back to the point where I was before rebooting.
Since you have already updated to Plasma 5.8.4, is this issue still reproducible with that version?
I'm reporting the problems using two different machines - a laptop and a desktop both running Linux Mint 18 KDE5 64-bit. I can't tell if the issue is gone. I had other problems so I'm not staying with the newer Plasma for production right now.
Since without that info this bugreport is useless, and most like the underlying issue fixed in 5.8 anyway, I'm closing it. Thanks for reporting the issue, however.