“post author” drop-down missing on Edit Post screen

I’m having a problem getting the “post author” drop-down (on the edit post screen) to appear on certain posts. This seems to occur in the following situation:

1. I, as admin, publish a post.

2. I, still logged in as admin, edit the post to assign it to a different user. For this step, the “post author” drop-down DOES appear.

3. If go back into editing the post, the “post author” drop-down DOES NOT appear. This is true if I log out/in as admin; if I log out and back in as the user to whom it was assigned; etc.

In other words, once I assign authorship to someone not me, I lose the “post author” drop-down, preventing me from editing authorship after that first change.

Any help appreciated!