displayname does not work with User Picker (multiple users)

Both ${displayname:<Field name>} and ${displayemail:<Field name>} doesn't work with custom field of type "User Picker (multiple users)".
In log, I see

/plugins/servlet/jiraxporter [xpandit.jiraxporter.a.f] Unrecognized user: 

for every User Picker (multiple users) custom field in the export. The problem occurs even if there's only one name in the field.

"User Picker (single user)" works ok with displayname.