This happens to the other notification that I have, for this topic: https://www.mediawiki.org/w/index.php?title=Topic:U9adjtb1wlq57een&action=history by user Ehtech2000: https://www.mediawiki.org/wiki/Special:Contributions/Ehtech2000

I've posted a test message below Ciencia Al Poder's message and now I can see Waanders message. That's wired.

Yeah, I now see both replies on that topic. Maybe something failed to update on redis/memcached, and the old cached data wasn't refreshed properly

This also happened to @jmatazzoni at https://www.mediawiki.org/wiki/Topic:U9pb2yqvw98167zo : I don't see his posts in a normal browser window, but in an incognito window I do.

As someone who was not following the topic mentioned in the OP, I'm pretty sure User:Waanders' reply was visible to me before too.

Also, @Ciencia_Al_Poder didn't you have a reply posted on the topic: using the word "head" throws error? It isn't visible and also does not appear in the topic's history. Maybe the revision was just suppressed making it invisible to me in history too.
Edit: The reply does appear on Project:Support desk's history page.

I think I have a good line on what his happening. When I'm logged OUT, I see the missing comments. When I'm logged IN, I don't see them. I confirmed this pattern on Mac Chrome, Safari and Firefox, using multiple accounts.

In T190429#4073710, @jmatazzoni wrote:

I think I have a good line on what his happening. When I'm logged OUT, I see the missing comments. When I'm logged IN, I don't see them. I confirmed this pattern on Mac Chrome, Safari and Firefox, using multiple accounts.

Ugh, yes, that's true, I can confirm that. However, the topic history doesn't reflect the new comment, being logged-in or in private browser, until someone adds another comment to it. I'm not able to see other instances of this problem on recent topics. I wonder if it was a temporal problem that has been fixed itself, or it's an intermittent problem.

Data about the messages in each topic is probably cached in the object cache, and a failure to update the cached data when the new message is posted can cause this behavior. If that's the case, the problem can easily affect other areas of the wiki, like pages displaying old versions, images not reflecting new dimensions after an upload, etc.

First weird finding: the updated version of the topic (6 comments: Kerry, Pigsonthewing, Joe, Kerry 3x) is cached in Varnish, but the version with only the first 2 comments (Kerry and Pigsonthewing) is what I get when I hit the app server directly. This happens both when I'm logged in and when I go to a URL like https://www.mediawiki.org/wiki/Topic:U9pb2yqvw98167zo?roan=1 in an incognito window. So somehow this topic has gotten screwed up either in the backend or one of its caches, but Varnish was never told to purge its version.

I posted a reply to that topic and that seems to have fixed it. The missing comments are back, and also appear in the history. I don't know what happened here, but most likely Flow's internal cache (WANCache) was populated wrongly somehow.

My ability to debug this was further impeded by the fact that logstash doesn't seem to capture DB queries in debug mode any more.

In T190429#4074096, @Catrope wrote:

My ability to debug this was further impeded by the fact that logstash doesn't seem to capture DB queries in debug mode any more.

Filed as T190455: Logstash no longer captures DB queries in debug mode