Wanted to document this in the event that someone comes searching.
I was injecting an Illustrator SVG into the DOM via JavaScript, and the animation for polylines would get to about 80%, and then immediately jump to 100%. After playing w/ CSS properties, I found the root of the issue.
Illustrator dumps out SVGs with a lot of linebreaks. I was escaping the carriage returns in JS like so:
Removing the carriage returns smoothed things out immensely and all is good in the world.