When annotation text consists of more than one line, the line spacing expands when the last character is "\n" so that only 2 or 3 lines of text are visible, even on a large monitor. Appending a space at the end of the string is an easy workaround, at least on Mac CC 2017. Perhaps line spacing can be unpredictable in other situations, as well.
Unfortunately Illustrator does not support drawing multi-line annotations out of the box, so we have to parse your string for "\n" and then draw each line individually, taking a guess about what the leading should be (based on the font size). It's not perfect but it's the best we can do.
Thank you, Garrett, for the explanation and also for implementing that! I found that multiple consecutive "\n" can cause problems, also, so if a plugin has a varying number of annotated text I just parse for multiple new-lines and all is well. I really appreciate the extra functionality you put into CORE!