I just found that this is a ridiculous issue that Quicken have been dragging for long time.
Specially with split transactions. if you copy and paste one (i.e. because you are entering a bonus payment that is coming apart of your regular salary) the original transaction will disappear, like you modified the original transaction instead of just copying and pasting, and then changing the one that you pasted.
Like I said, this is ridiculous and Quicken need to resolve this like right now.