Certain Subsite Posts Not Appearing In Recent Global Posts Feed

Hi. I've spent the past few hours troubleshooting why certain posts from one subsite are not appearing in our Recent Global Posts Feed. I've searched extensively, and chatted with Luís during Live Support hours, but neither of us could find similar reports. So here goes...

One of our users reported that some of her blog posts were not showing up in our network-wide RSS feed. We use the Recent Global Posts plugin to generate the feed at:
http://tripawds.com/wp-content/recent-global-posts-feed.php
and/or:
http://tripawds.com/feed/globalpostsfeed

The feed for the specific site having issues with posts not showing in the global feed is:
http://purrkins.tripawds.com/feed/

All of these feeds validate fine. Note, however, that a recent post from purrkins on March 29 is in that site's feed, but it is not in the global Tripawds feed.

For the record, purrkins.tripawds.com is a Pro Site and we are running Pro Sites with the original "Supporter" gateway.

Recent posts from purrkins that are missing from the global feed do show up in our Blogs Directory and in the Recent Posts Widget we display around the main site.

Post Indexer stats for the purrkins site show only 12 posts indexed, of 26 published posts. All the posts are published, public, and not password protected.

I have ruled out any plugin conflicts by performing the usual troubleshooting procedures on both the purrkins site and the main site. I have re-saved permalinks on both the purrkins site and the main site. I have rebuilt the Index both for the purrkins site by itself and the entire network, multiple times, after each of these steps.

And, here's the kicker: I published a test post as the purrkins user on her site and it now appears in the feed while her post from two days ago still does not.

After my last index rebuild, a few of the purrkins posts were listed in the Recently Indexed Posts according to the Post Indexer stats, but none of these were the most recent post for March, 29. Reviewing that and other posts not in the feed did not shed any light on the issue, other than the fact some of them include emojis in the post content.

I have reviewed entries and compared data in the database tables for the subsite and wp_network_posts only to confirm that 12 of the site's 26 posts got indexed, without identifying any sort of pattern.

During my troubleshooting and index rebuilds I enabled and monitored PI_CRON_DEBUG but noticed no unexpected anomalies.

I have checked the database and optimized tables to remove any overhead. I just don't know what else to do next to try and figure out what is going on, so I'm turning to the experts here.

Why would Post Indexer only index some of the posts on a certain site?

What further steps can I take to troubleshoot this issue? What other information do you need from me? I'm attaching some screenshots for clarification, and have granted Support access for the Tripawds network admin.

Thanks in advance for any help!