Nonsense (and/or undocumented) "clean:" before filename in grep output

It has started happening to me recently, than when I do:

> grep -R something

besides the normal expected output which is of the form:
  filename: match
  filename: match
  ...

I also get some lines like this:
  clean:filename: match

where the word "clean" appears literally.

The matches that are shown in this form also appear in the normal form without the "clean:" prefix, so in total they show up twice.

And I have checked that I have no files whose filename actually starts with "clean:"

I have checked the manpage for grep and there's no occurrence of the word "clean", so if that "clean:" is supposed to actually mean something (besides being counterintuitive and being a shitty inconsistent format, and besides the fact it wouldn't make sense to show the matches twice), it's not documented.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: grep 2.25-1~16.04.1
ProcVersionSignature: Ubuntu 4.4.0-135.161-generic 4.4.140
Uname: Linux 4.4.0-135-generic x86_64
NonfreeKernelModules: nvidia_uvm nvidia
ApportVersion: 2.20.1-0ubuntu2.18
Architecture: amd64
CurrentDesktop: Unity
Date: Thu Sep 27 17:43:46 2018
InstallationDate: Installed on 2013-10-11 (1811 days ago)
InstallationMedia: Ubuntu 13.04 "Raring Ringtail" - Release amd64 (20130424)
SourcePackage: grep
UpgradeStatus: No upgrade log present (probably fresh install)