When collating a subset of a tag with:

{% assign posts_collate = site.tags.WIP %}
{% include JB/posts_collate %}

the generated list will not show the correct month and year when the posts which are not in the subset cross the month or year boundary.

As far as I can tell this is caused by the following code in posts_collate:

{% capture next_year %}{{ post.previous.date | date: "%Y" }}{% endcapture %}
{% capture next_month %}{{ post.previous.date | date: "%B" }}{% endcapture %}

post.previous is not the actual previous post in a subset. The correct previous post would be the next post in the array.