Bug overview description.
Example:
install Ublock origin and make sure the "Adguard base filters" are enabled on the dashboard. This causes any post thumbnail which has tags starting with "advertisement" to be hidden. This also renders it un-clickable without reading the page source.

Since this is primarily a client-side issue, I don't know if it is possible to fix with a page code tweak.
If it *CAN'T* be fixed, it would be nice if it could be listed as a known issue somewhere obvious. It took a good 30 minutes to figure out what was randomly breaking one out of a thousand posts.

What part(s) of the site page(s) are affected?
index, search results and presumably any page with thumbnails.
Other pages may be affected depending on how a particular filter works.

Can you reproduce the bug every time?
Yes.

What steps did you take to replicate this bug?
Tried multiple browsers

Oh adblock!

Sounds like you should whitelist e621.net ;)

This is a known issue. This is expected behaviour for adblockers.

Because this is out of our control, I recommend whitelisting #content > #post-list > .content-post > div > .thumb > *

In uBlock Origin, you can do this by adding the custom rule e621.net#@#.thumb and e621.net#@#.preview

Please note that using extensions and scripts that modify the site are unsupported. This information I have put here is purely of convenience and comes with no warranty or support.

If you wish for this to be fixed, please contact your adblock provider and have them add the specific whitelistings for e621 and e926.

Quick note:
It is quite trivial to use the whitelist to fix this, but the main concern is getting to that point, as the cause wasn't immediately apparent.

Would it be appropriate to add the above technical note to advertisement's wiki page? It wouldn't be seen much, but I can't seem to find a dedicated bug list anywhere.

Working as intended then, huh?

Yes, adblockers will block elements with the alt text containing the word advertisement.
This is expected behaviour from adblockers.