PageCuration sometimes fails to tag articles, with the popup message "failed to apply tags to the page".
However in cases of failure it still marks the pages as reviewed, and (if applicable) still creates the AfD page.
A few reports of this bug from various editors at https://en.wikipedia.org/wiki/Wikipedia_talk:New_pages_patrol/Reviewers#curation_tool_error:_failed_to_apply_tags_the_page
I was not able to reproduce this bug (in Firefox). I tried several different sequences of tagging and reviewing (with different tags) and wasn't able to get the error. Reliable reproduction steps are needed.
Tried reproducing in Chrome and couldn't reproduce: https://en.wikipedia.org/w/index.php?title=The_Fa%C3%A7ade&type=revision&diff=835801399&oldid=835725163.
The error message being displayed in the UI comes from modules/ext.pageTriage.views.toolbar/ext.pageTriage.delete.js, line 767.
I saw that articles mentioned on https://en.wikipedia.org/wiki/Wikipedia_talk:New_pages_patrol/Reviewers#curation_tool_error:_failed_to_apply_tags_the_page e.g. Dilip De, Dilip K De, and Miltiadis Karatzis were eventually deleted and marked with the tags mentioned.
The issue is not possible to reproduce in betalabs:
Applying tags from 'Speedy deletion' - no warning ("failed to apply tags to the page") is displayed. Non-existing in betalabs template will be added to the page. The page itself will be displayed as Reviewed on Special:NewPagesFeed, and not with the trashbin icon as in enwiki.