Relative pauses requires the text speed to not be maxed out. It's relative to the pause between each character. If the pause is 0 (the text speed is maxed out), then there's no possible pause because the multiplication to make it relative results in 0.
Try reducing the text speed in the preferences. You can see at 0:56 on the bottom left of the preferences screen.
I consider this is a UX problem and not a bug. What do you suggest for me to fix it? Maybe a message that appears under the "Sentences" bar when the Text Speed is maxed out warning just that? Or maybe nearby the "Relative pauses" radio button? Or maybe you have a better idea? For ease of install, I don't want to require more than the use for integration in settings.