_version number incremented, but updates not reflected

We have came across a weird situation where we see the _version number incremented, but the actual document is not updated at all. We noticed that for said failed writes, the _version number is higher than usual (we normally see _version = 3, but for failure cases we see it go up to 4 or 5)

To allow us to further analyze the issue, we would like to know if _version is incremented only by external write requests? Or does internal retries trigger an increment to the _version number as well?