Wildly enough, the title describes this problem quite well.
If the column name alias I choose does not contain a lowercase character, then the column width will be just barely too small. And this is for tiny column names too, not even long ones.
Here is a screenshot of the proof.
Now, this has a catch -- the lowercase only corrects the problem if the alias does not also contain numbers. If it does, you are back to being just barely too small.
Here's more proof.
I am using Firefox (133.0.3, 64 bit version) and I am using Windows 11. Don't know what other info to provide, since this is fairly easy to recreate.
Here is a link to the query that made the screenshots. Feel free to fiddle around and try it yourself.
https://data.stackexchange.com/stackoverflow/query/1881440
I should also mention, I think it is literally 1 pixel off. If I drag the column open by even a single pixel, it corrects itself. Which leads me to believe that that this is just an off-by-1 error or something.
Finally, I would think that expected behaviour would be to just make the column width perfectly match the column length. If the user thinks a column is too large, they can shrink it themselves imo.

