What I'm hearing is this has been fixed since at least Hirsute.
** Changed in: lxqt-session (Ubuntu)
Status: New => Fix Released
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https://bugs.launchpad.net/bugs/1804329
Title:
** Summary changed:
- lubuntu 19.04 daily - pcmanfm-qt (dolphin..) moves window position on opening
device, folders or adjust size of windows (when on 2nd display only with
vertical placement)
+ lubuntu 19.04-20.04 daily - pcmanfm-qt (dolphin..) moves window position on
opening device, folders