Valid UTF8 characters wrongly detected as invalid

I have a .sql text file which contains these two valid 3-byte utf-8 characters: 😍😜 (it's two smilies).
Not only Gedit doesn't display them correctly, it issues the warning:
"There was a problem opening the file ...
 The file you opened has some invalid characters. If you continue editing this file you could corrupt this document."
with the option to retry with a different character encoding.

The characters are then shown in red background as follows:
  \ED\A0\BD\ED\B8\8D\ED\A0\BD\ED\B8\9C

They are valid UTf-8

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: gedit 3.10.4-0ubuntu10
ProcVersionSignature: Ubuntu 3.19.0-30.33-generic 3.19.8-ckt6
Uname: Linux 3.19.0-30-generic x86_64
ApportVersion: 2.17.2-0ubuntu1.5
Architecture: amd64
CurrentDesktop: Unity
Date: Mon Oct 12 21:09:34 2015
InstallationDate: Installed on 2013-10-11 (731 days ago)
InstallationMedia: Ubuntu 13.04 "Raring Ringtail" - Release amd64 (20130424)
SourcePackage: gedit
UpgradeStatus: Upgraded to vivid on 2015-08-15 (57 days ago)