Updating basictex did not trigger format rebuild

[Karl Berry]

    In the next several months I'll concentrate on mathematics rather
    than TeX,

What a concept..

    so let me pass on what I know and let all of you handle this
    problem.

I don't think we can.

    For BasicTeX, but not MacTeX, I uncheck the option "make
    format files" in the TeX Live install script. 

Well, if I'm understanding correctly, that is exactly the issue,
Dick. By unchecking that option you are telling TL to leave the formats
alone. Therefore it does so. I suggest leaving it checked for BasicTeX
too.

    I do not build formats in BasicTeX because I want this package to be as
    small as possible. 

basictex*.pkg is 76MB. I doubt a few .fmt files will increase it
appreciably, especially since presumably they are compressed as part of
the .pkg file.

    never had reports of trouble after updating the distribution. 

I surmise that the intersection of (a) people using BasicTeX
and (b) a format change being noticeable to the user
has heretofore been empty. Usually, the formats that get built at
install time are fine.

    Reports of trouble that emerged yesterday suggest a
    problem only on Catalina,

I can't imagine how the OS version could be relevant.

    and possibly only for BasicTeX.

Right, because that is where the option is unchecked.

Happy mathematicizing,
Karl