Appears to be to do with RightsLogFormatter::getMessageParameters, Language::listToText, and $newGroups array keys not starting at 0 on some entries. Moving to MediaWiki product.

This hack fixes it for me:
diff --git a/includes/logging/LogFormatter.php b/includes/logging/LogFormatter.php
index 8c1e294..26368b4 100644

• a/includes/logging/LogFormatter.php

+++ b/includes/logging/LogFormatter.php
@@ -1108,7 +1108,7 @@ class RightsLogFormatter extends LogFormatter {

        $params[3] = $this->msg( 'rightsnone' )->text();
}
if ( count( $newGroups ) ) {

• $params[4] = $lang->listToText( $newGroups );

+ $params[4] = $lang->listToText( array_values( $newGroups ) );

} else {
        $params[4] = $this->msg( 'rightsnone' )->text();
}

Changed summary, as it's happening too on commons.wikimedia.org:

https://commons.wikimedia.org/w/index.php?title=Special%3ALog&type=rights&user=Trijnstel&page=User%3AShinePhantom&year=&month=-1&tagfilter=

22:42, 14 November 2012 Trijnstel (talk | contribs | block) changed group membership for User:ShinePhantom from OTRS member, autopatroller and file mover to OTRS member and [missing](OTRS members are already autopatrolled).

Regards.

Gerrit change 33878

• Bug 42411 has been marked as a duplicate of this bug. ***

• Bug 42418 has been marked as a duplicate of this bug. ***

Merged by Roan.

• Bug 43538 has been marked as a duplicate of this bug. ***

I see this "Resolved - Fixed" and "Merged" in comment 7 but the problem still happens. Is the code working in the server or awaiting commit?

From meta rights' log:

• 11:05, 31 December 2012 Vituzzu (talk | contribs) changed group membership for User:Vituzzu@itwiktionary from CheckUser and patroller to (done)

Thanks (& happy new year)

It's fixed, but WMF haven't updated their version of the software to include the patch yet.

You can see their schedule here: https://www.mediawiki.org/wiki/MediaWiki_1.21/Roadmap
(It should be in 1.21wmf7)