Removing a user with a dot in its name from a group does not work

We have XWiki Virtual farm. User accounts can be created either directly or through XWiki JDBC Authenticator (https://github.com/xwiki-contrib/xwiki-authenticator-jdbc).

When I create a new group local to the virtual wiki and add users, it all works. But when I try to remove a user from the group. It hides the user upon clicking "X", but upon another action (filter, close, save) the user is back in the group - thus the user was not truly removed. I cannot remove any user from the group.

When the "remove" link in group interface is clicked, XWiki throws NPE in the server log (attached).

Note that this behavior occurs now on 5.1 but it also occured before on 4.4.1.