xterm sources:
    ftp://ftp.invisible-island.net/xterm
I agreed that it's more likely to be something else than the version.

On my Linux virtual terminal, I get your observed incorrect at-2 unpatched
behavior with zsh -df, but with just zsh I get the correct (past 2)
behavior even without the patch.

zsh version is the 5.7.1 release.  I can try your patch later today.

You should definitely check something rxvt-like as there are several
terminals derived from rxvt besides rxvt-unicode (aterm, eterm, mrxvt).
rxvt-unicode is at http://dist.schmorp.de/rxvt-unicode/Attic/