Yes likely a macOS/Firefox bug. Please note the bug thread title indicates in has to do with bbcode, but it does not. I can also just copy three lines which have a line feed and the line feed is lost upon pasting.
I attached an example.mp4 for you to see ny exact steps.
I cannot say whether it is a bug on the part of FF. I can only confirm that this is also the case under Windows with FF. Texts copied from Chrome, vivaldi and opera are correctly inserted into the editor.
If I copy the text into Notepad++, the formatting is correct.
Hey @Ralle, I had a hard time publishing a code submission in the Lua sub-forum today, because pasting code into a post would remove all blank lines from it. Of course, the code was intentionally formatted this way, so I had to manually re-add blank lines after pasting, which was very tedious...
Copy-pasting from VS Code removes line breaks. Copy-pasting from Notepad++ does not. As you said.