This can happen if someone goes forward in time on the lineup page. Due to how various parts of the site work, going forward to a future date wrote a new lineup to the database for that date. So if you went to the lineup page, went forward a bunch of dates, and now are setting your lineups, you are actually modifying all the lineups you “set” in the future by visiting a future date on the lineup page.
I’ve modified this behavior this morning so going forward, if you go to a future date, lineups are only committed to the database if you make a change. Otherwise the system will look at the most recent lineup slot for every player in your lineup as best it can.
I’m continuing to monitor this.