This seems to be working as expected again! @IllusionaryOne and I just tested it.
Seems to only be broken for some users. I’ve been tolled that Twitch is working on a fix for this though. The temporary fix is to get the user to follow your bot’s account.
Edit March 20:
Seems like twitch has changed their whisper system and the user has to whisper the bot before it can whisper that user. If the user is following the bot account this will not be an issue.