Yes, annoying. I assume that you are describing the smaller space on the right of the caption, compared to the left, so it is not centered.
It is complicated, since the spacing varies by font size. And it is most apparent in a 9x16 shape. I tested in a 1920x1080 with font size at 66. (I did not see till later that you are using 60.) All is good up to 43 characters. At 43, right and left "margins" are the same. At 44, the left side of the caption text no longer "moves," the added characters move to the right, and there is less space to the right. You can add as they happily move right off screen. If you delete them one at a time, the margin is the same at 43. But at 44, it is not.
At font size 40, (just the one I picked arbitrarily), the left side stops moving at 73. At 74, the margin is visibly off.
The workaround is to use one fewer letters and/or reduce font size. But I agree that font size 60 (I often use 66) is a better size for 1920x1080 type material. But it is more of a problem when going to portrait mode.
Regarding the future, the public Beta has a new caption workflow that is using the essential graphics engine. I believe this will solve many of the problems with captions and add new features. I have not tested this issue in the Beta. But because of those changes, I don't expect any changes in the current caption system. Note that the Beta captions are not ready for primetime, although many of the basic functions are now available.