Hey sorry, just saw this!

Hm, If I understand correctly, the diff you linked to has the same characters incorrect? '�ир!'. Is that right?
If so, this is a problem with how the string was saved in MediaWiki, not EventStreams delivery. EventStreams is giving you the string as MediaWiki had it.

Please correct me if I am wrong. :)

In �ир missing character was м, in запро� it was с, and in Хакур�й it is е. The article in the linked diff was called Миру — мир! (скульптура), EventStreams emitted that title with a missing м. The same error was happening for other fields (Хакур�й is from a username, запро� is from a comment).

Today, in November 2019, I can’t really say if this still persists; maybe others might chip in. When I filed the task, this was much more frequent, maybe there were some unknown changes that fixed this.

Hm, right, but what I mean is, the missing м looks like it is actually missing from the saved content:

curl -s 'https://ru.wikipedia.org/w/api.php?action=compare&fromrev=93764185&torelative=prev&format=json' | jq .

{
  "compare": {
    "*": "<tr>\n  <td colspan=\"2\" class=\"diff-lineno\">Строка 1:</td>\n  <td colspan=\"2\" class=\"diff-lineno\">Строка 1:</td>\n</tr>\n<tr>\n  <td colspan=\"2\" class=\"diff-empty\">&#160;</td>\n  <td class=\"diff-marker\">+</td>\n  <td class=\"diff-addedline\"><div>{{User:IluvatarBot/Подозрительный источник|Миру — �ир! (скульптура)|93761090|93764184| livejournal.com|Ksenya1|1530789469}}</div></td>\n</tr>\n<tr>\n  <td class=\"diff-marker\">&#160;</td>\n  <td class=\"diff-context\"><div>{{User:IluvatarBot/Подозрительный источник|Ярославское восстание|93668421|93763600| livejournal.com|Tpyvvikky|1530787434}}</div></td>\n  <td class=\"diff-marker\">&#160;</td>\n  <td class=\"diff-context\"><div>{{User:IluvatarBot/Подозрительный источник|Ярославское восстание|93668421|93763600| livejournal.com|Tpyvvikky|1530787434}}</div></td>\n</tr>\n<tr>\n  <td class=\"diff-marker\">&#160;</td>\n  <td class=\"diff-context\"><div>{{User:IluvatarBot/Подозрительный источник|60-й отдельный бронепоезд|93759489|93759498| forum|Семен Владимиров|1530768880}}</div></td>\n  <td class=\"diff-marker\">&#160;</td>\n  <td class=\"diff-context\"><div>{{User:IluvatarBot/Подозрительный источник|60-й отдельный бронепоезд|93759489|93759498| forum|Семен Владимиров|1530768880}}</div></td>\n</tr>\n\n<!-- diff cache key ruwiki:diff:wikidiff2:1.12:old-93763603:rev-93764185:1.9.0 -->\n",
    "fromid": 7404545,
    "fromrevid": 93763603,
    "fromns": 2,
    "fromtitle": "Участник:IluvatarBot/Badlinks/raport",
    "toid": 7404545,
    "torevid": 93764185,
    "tons": 2,
    "totitle": "Участник:IluvatarBot/Badlinks/raport"
  }
}

So EventStreams is giving you exactly what MediaWiki has. This wouldn't be a problem with EventStreams, it happened somewhere in MediaWiki.

I think you misunderstood what the purpose of that diff was. The bot in the diff consumes EventStreams data and posts the result. These kinds of omissions were present in EventStreams data, not in MediaWiki. To prove this, my Discord bot regularly encountered the exact same problem, despite not touching MediaWiki at all in the process.

Ah! I did misunderstand that, thank you. Ok I think I get it now.
So the bot consumed an event for revision 93764184 create (right?), which is for a page with the title with мир:

curl -s 'https://ru.wikipedia.org/w/api.php?action=compare&fromrev=93764184&torelative=prev&format=json' | jq .compare.totitle
"Миру — мир! (скульптура)"

but the м missing when the bot got the title from EventStreams.
That does indeed sound like a problem with EventStreams.

I can’t really say if this still persists

Yeah, I'm not sure either. I just created a test page on test.wikipedia.org with these characters and got the correct ones in EventStreams, so I can't reproduce directly.

Bug was not in specific symbols. Periodically (not always!) EventStream did a defect in random characters of different writing (not only in Cyrillic). One time an error is in "м", and another time "м" is normal, but error is in "y" or "ф".

See another very old screenshot. "М" is normal: https://imgur.com/a/zQHCitr

Bug disappeared a few months after submitted of this task. Now that bug does not exist.
Sorry for my English.