The Texinfo manual clearly states: "Only the current font is checked for ligatures and kerns; neither special fonts nor entities defined with the 'char' request (and its siblings) are taken into account."

This limitation seems the wrong default.  If kerning were done by default for .char-defined characters, users who want to disable kerning before or after such a character could do so with \&.  Instead, currently such kerning is inactive by default, and there is really no way to tell groff to turn it on.

Two real-world cases where this matters:

• Bug #59839: the proffered rfc1345.tmac is a list of nearly 1700 character aliases defined with .char.  With the current restriction, no character specified with one of these aliases will be kerned.
• Bug 58930 comment 9

On the email list, I proposed changing this behavior (http://lists.gnu.org/archive/html/groff/2021-01/msg00031.html), and while responses were extremely limited, no one opposed the idea.