It is not a bug.
Apple removed the option in Catalina.
I called Apple Support today and the first tech who shared my screen could not understand why I can no longer adjust the column width and change the Message List font style in Mail Preferences but can only change the font size (the only one that comes with Mail v. 13.0.
So he handed me over to a Senior Advisor who also shared my screen.
He told me other Mac users have complained about the same issue but no solution was found yet.
Then, after the Senior Advisor directed me thru doing a live Quick Time Screen Recording and having me show the problem on my iMac (it's the same width/font issue for Apple Mail v. 13.0 on my MacBook Pro, so he knew it was a Catalina issue) and having a log file created to send to "Apple's Engineering Dept.," I decided to deselect Column View Layout on the Mail toolbar and found that I could successfully change both the font style and font size in Message List to what I liked.
The Senior Advisor then told me that he researched the issue while I was doing that and he read me an internal report that confirmed Mail in Catalina no longer supports Column View Layout or changing the font style in that mode.
Like most of you, I prefer to use Column View and am not pleased with not having the option to continue to use Column View the way I had it set up in every other version of Mac OS I have used previously, which for me goes back eight years.
Finally, the Senior Advisor provided me with this same website others in this forum referenced to give feedback to Apple to let them know I want to be able to use Column View the way I could before.
He told me if he hears of a way to adjust the column widths and change the font in Column View, he will contact me; however, he did say that because it is not technically a malfunction in Apple Mail but rather a change in the program, he doubts he will have a solution and apologized. He told me he agrees that this was not a positive change in Catalina and Apple should offer the option to use Mail the way I (and now I can say, "We" after reading all the posts in this thread) prefer.
So....I will send my feedback thru this link, below, and "hope" it makes a difference.
https://www.apple.com/feedback/mail.html