See tdf#104717 for a ~similar problem (involving PRIMARY and CLIPBOARD) - but it’s for KDE and its “Synchronize contents of the clipboard and the selection” setting under “Clipboard contents”. Not sure if there exists something like that.
OTOH, you wrote:
I don’t want to use GTK 3 because it’s usage by LibO is too buggy
but you didn’t mention if you had tried GTK 3 (temporarily) to test if the problem still is there; nor did you mention if you meant GTK plugins of LibreOffice or system GTK; which LO version you use; and even which OS you use…