For instance this simple sheet, with one conditional formatting rule over the whole A column:

When copying cell A4 and pasting it in A5, the conditional formatting rule is changed to 2 rules:

This behaviour exist in both Freeoffice 2021 as well as the full Office 2018.
I would not expect the conditional formatting rule to change at all. That's the way it works in Excel or LibreOffice.
Is it a bug?