Beta 9.1 - Edit post to tag another user messes up other tags

Before I log this, was this an update from 4.x?

Yes, I have upgraded from 4.X when the 1st beta came out

Sorry, to clarify, is the entry itself that you are editing, an upgrade from 4.x?

It happens on both upgraded or fresh installs:

The post above was made, and I edited it to add another user. The first mention is gone; it's just a link to the user.