Which message key is this? (You can find out using ?uselang=qqx.) The message I see in my local i18n file is correct:

"flow-notification-newtopic-bundle": "{{PLURAL:$1|One new topic|$1 new topics|100=99+ new topics}} on <strong><span class=\"plainlinks\">[$3 $2]</span></strong>",

In T137296#2367303, @Catrope wrote:

Which message key is this? (You can find out using ?uselang=qqx.) The message I see in my local i18n file is correct:

"flow-notification-newtopic-bundle": "{{PLURAL:$1|One new topic|$1 new topics|100=99+ new topics}} on <strong><span class=\"plainlinks\">[$3 $2]</span></strong>",

This key is from the old system (notice the markup and the links).

It's notification-bundle-header-flow-new-topic, and notification-bundle-header-flow-new-topic-user-talk seems to have the same issue.

Change 293496 had a related patch set uploaded (by Catrope):
Update messages referring to topics and replies for 99 changes in Echo

https://gerrit.wikimedia.org/r/293496

Change 293496 merged by jenkins-bot:
Update messages referring to topics and replies for 99 changes in Echo

https://gerrit.wikimedia.org/r/293496