Hi Robert,

Confirmed here; it seems that pdftex.web L.3779

@d auto_kern == explicit

is the culprit.

In your proposed MWE, when the function adjust_interword_glue()
is called, the tail node is \kern1pt thus (type=kern_node) and
(subtype=explicit).  However, "@d auto_kern == explicit" is set,
it falls through the following conditions.

    else if (type(p) = kern_node) and
            (subtype(p) = auto_kern) and
            (save_tail <> null) then

When I change that line to

@d auto_kern == 3 {|acc_kern| + 1}

the problem is gone.  I'm not sure whether there is some
side effect.