When I choose to upgrade swig from the TUI, I get the incorrect message
------------------------------------------------------------
swig4.0 (remove, 4.1.0-0.3) will be automatically removed because of dependency
errors:
------------------------------------------------------------

with nothing else.

With more details:

I currently have

i   swig                       4.1.0-0.3                4.2.0-1

Over this line, I type '+', so that I get:

aptitude 0.8.13 @ cventin                Disk: -184 kB               DL: 1429 kB
[...]
iu  swig             +5509 kB  4.1.0-0.3                4.2.0-1

Now, I type 'g', and I get:

------------------------------------------------------------
swig (upgrade, 4.1.0-0.3 -> 4.2.0-1) will be upgraded from version 4.1.0-0.3 to
version 4.2.0-1.
------------------------------------------------------------

Over the swig4.0 line, I get:

------------------------------------------------------------
swig4.0 (remove, 4.1.0-0.3) will be automatically removed because of dependency
errors:
------------------------------------------------------------

with nothing else. Without any error message, this doesn't make sense.

Then, when I type ':' over the swig4.0 line, this message changes to:

------------------------------------------------------------
swig4.0 (remove, 4.1.0-0.3) was installed automatically; it is being removed
because all of the packages which depend upon it are being removed:

  * swig (upgrade, 4.1.0-0.3 -> 4.2.0-1) depends on swig4.0 (>= 4.1.0-0.3)
    (provided by swig4.0:i386 4.1.0-0.3)
------------------------------------------------------------

Now this is almost correct. I don't understand the ":i386". This is
a configured foreign architecture on this machine, but this package
is not installed:

cventin:~> dpkg -l | grep swig
ii  swig                                           4.1.0-0.3                            all          Generate scripting interfaces to C/C++ code
ii  swig4.0                                        4.1.0-0.3                            amd64        Generate scripting interfaces to C/C++ code

Note that with the currently installed swig package:

Package: swig
Version: 4.1.0-0.3
[...]
Architecture: all
Replaces: swig2.0
Depends: swig4.0 (>= 4.1.0-0.3)
Suggests: swig-doc, swig-examples
Conflicts: swig2.0

and with the new version:

Package: swig
Version: 4.2.0-1
[...]
Architecture: amd64
Replaces: swig4.0
Depends: libc6 (>= 2.34), libgcc-s1 (>= 3.0), libpcre2-8-0 (>= 10.22), libstdc++6 (>= 13.1)
Suggests: swig-doc, swig-examples
Conflicts: swig4.0

Note that there was a similar bug in the past (2005, fixed in 2016):

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=342835

with title:

  aptitude: "X will be automatically removed because of dependency errors:"
  then no errors shown

I cannot reproduce this issue on another machine, which doesn't have
a foreign architecture. I get just after 'g' and putting the cursor
over the swig4.0 line:

------------------------------------------------------------
swig4.0 (remove, 4.1.0-0.3) will be automatically removed because of dependency
errors:

  * swig (upgrade, 4.1.0-0.3 -> 4.2.0-1) conflicts with swig4.0
------------------------------------------------------------

However, AFAIK, it will be automatically removed not because of the
conflict, but because the new swig package no longer depends on it
(on both machines, swig4.0 is marked as automatically installed).

This is a different message from