I can repeat what you see every time.
The Mail window is a little buggy, I think.
Here's how I fix it, and this works every time that I tried:
Drag the right side of the Mail window to expand the total size to the right, maybe 3 or 4 inches, maybe more. Release the mouse button, and the extra right bar should appear with the small dot in the middle. If you don't see that extra bar, drag the window size a little larger, until you see that extra bar after releasing your mouse button. Then, you can double-click, or drag that dot toward the left.
I tried this a number of times. If the window is too small, side-to-side, then the operator bar to control that third column will disappear every time that I tried. Your Mail window needs to be wider for all of the window functions to stay active.
Again, not what I expect to happen, and it's (I think) a bug in the Mail software. Might be fixed in a future update.