I have encountered this a few times, and it isn't clear to me why.

What happens is that I'm making miscellaneous edits, and then, when the edits go through, an additional edit is created, which involves 'adding' an empty annotation.

The following edits are the three most recent times it has happened. There may be more that I haven't noticed:

Edit #86453004 - Add release annotation
Edit #86235316 - Add release annotation
Edit #86235166 - Add release annotation

I've tried hard to reproduce it, and I simply can't, but it has happened to me today and in the last week as above. 

I thought it might be that I had accidentally clicked in the annotation field, before making a few edits. I tried this process, and didn't manage to reproduce it. I also  tried just clicking in it, but because that didn't create an edit as such, it wasn't possible to perform that operation in isolation.

To be clear, these are releases that had no annotation prior to these edits. (I know this because I previously removed the rather spurious "very rare" annotations which previously sat there.) The second edit above is a bit different because I cancelled the edit before it went through, but it was also empty before that edit was made. 

I'm not sure how to reproduce this, as it seems to be unpredictable, albeit the result is consistent: an edit is created to 'add' an empty annotation, where it was empty before.

It's unclear to me if this would crop up on a non-empty annotation, but if so, it would potentially be quite a serious bug, as it would remove contributions that should not have been removed.

I've looked back about 30 pages of my editing history and can't find any more of these edits, although there could be more.