Same thing for me. No problems before, everything worked fine earlier today, and now all of a sudden my minions keep disappearing. I've never experienced this bug before and I was totally confused at my suddenly disappearing minions.
What comes to the comments about mounting and changing mounts too fast, that is not it at least for me.
This bug is really annoying. I'm playing through LW4 and mounts are kind of a requirement - to be able to change them on the fly, so to speak. And bam, my minions keep disappearing. Sigh. Hopefully they fix this asap.