I had a Gather collection with 1 article in it, created a couple of weeks ago. Today I added some 8-9 other articles to it (using the search bar), and they showed up fine, but after clicking "save changes": 1. I got no visual response that it happened; 2. refreshing the page after a long minute resulted in a now empty(!) collection. The collection URL is this: https://he.m.wikipedia.org/wiki/%D7%9E%D7%99%D7%95%D7%97%D7%93:%D7%9C%D7%A7%D7%98/id/1/100wikidays
I can't replicate this so far.... and cannot find any exceptions in logstash.
Not sure how to debug this without understanding the events that occurred leading up to it.
It's possible a one off resulting from enabling the extension on hewiki.
Can you check Special:Log to see which events happened for you related to Gather leading up to this?
@Ijon did you lose any other data across the site?
A maybe related bug that I can consistently reproduce (on vagrant, with the mobileweb role, with a normal broser) is: create new empty collection -> add a bunch of pages via the search box -> save -> still empty