If Apple Reminders do in fact add the extra lines, then there isn’t anything we can do - you’d have to file a bug report with Apple.
From our perspective, we can’t really “strip” preceding empty lines because many people do actually copy text with empty lines before or after when reorganizing paragraphs, and stripping it would be a much bigger annoyance.
I suspect it “works” between Apple apps because they have some kind of workarounds for each other (or have specific parsers that understand that the input came from Apple Reminders and performs the necessary stripping).