Changing a text layer's Leading affects the whole block of text
It is impossible to set the leading of individual lines in a block of text. For example, if I have three lines of text, if I put the cursor on the first line and adjust the Leading, the line spacing of all three lines changes (I have line breaks/returns at the end of each line - there is no wrap). I have tried selecting the entire line, selecting two lines - pretty much every kind of selection - but adjusting the Leading ALWAYS affects ALL the text. It's impossible to adjust only one line's Leading.
We do not have any per-line properties in Pr graphics currently. The same is true for Paragraph Alignment. But it's a good request and I have submitted an internal request for you.