There appears to be no size control on emoji's when in print view. See https://www.phpbb.com/community/viewtopic.php?p=14652986#p14652986 for an example.

I copyied from styles/prosilver/theme/common.css the following

.emoji {

    min-height: 18px;

    min-width: 18px;

    height: 1em;

    width: 1em;

}

into styles/prosilver/theme/print.css and it resolves the problem.