Reproduced and explained:

https://en.wikipedia.org/w/index.php?title=Wikipedia:Sandbox&oldid=1063957769

The section you're referring to has no content. It can thus not be subscribed to.

Not even text content suffices: The section must contain a signature to become subscribable.

https://en.wikipedia.org/w/index.php?title=Wikipedia:Sandbox&direction=prev&oldid=1063957736

I'm afraid this works as intended.

@ToBeFree, thanks for the explanation. In that case, I'd still like to be able to subscribe to [[en:WP:AE]] sections, and the subsections contain signatures, so I have renamed this to "Ability to subscribe when no signature in section but signature present in subsection".

Why it works like this:

1. We only allow subscribing to level 2 headings, because allowing sub-sections would require a complex user interface (particularly in managing subscriptions when multiple subscriptions with different states could overlap), so we gave up on it.
2. We require the section to have comments, because we track sections across renames/moves by using the author and time of the oldest comment.
3. We don't count comments in sub-sections, because we were worried that if we counted them, then merging sections (e.g. https://en.wikipedia.org/wiki/Wikipedia:Village_pump_(technical)/Archive_193#Performance_issue_with_syntax_highlighting) would cause subscriptions to be lost, if the added sub-section was older.

We could probably handle #3 there differently, and remove this limitation. There are already some obvious cases where it doesn't work well.

I think that if T299657: Ensure notifications are sent when someone closes a discussion you are subscribed to was implemented, in such a way that we'd generate a notification when sections are merged that way, then we could just count the comments in sub-sections too, which would make these sections subscribable.

Change 811348 had a related patch set uploaded (by Esanders; author: Esanders):

[mediawiki/extensions/DiscussionTools@master] Fix fetching of oldest comment in a thread

https://gerrit.wikimedia.org/r/811348

Change 811348 merged by jenkins-bot:

[mediawiki/extensions/DiscussionTools@master] Fix fetching of oldest comment in a thread

https://gerrit.wikimedia.org/r/811348