[not really an answer yet, sorry but your question may be closed by overzealous reviewers]
I can reproduce the bug with dash 0.5.8 from debian 9.8 stretch.
The current dash code from git.kernel.org doesn't show the bug.
Here is a simplified testcase:
for s in HUP INT; do trap ' trap -- - '"$s"' EXIT || exit "$?" kill -s HUP -- "$$" || exit "$?" ' "$s" done kill -INT $$
$ dash dash-bug dash-bug: 3: /home2/ahq/dash-bug: Syntax error: EOF in backquote substitution
Modifying the length or content of the quoted code may produce different and interesting corruptions, with random bytes appearing out of nowhere.
It remains to be seen if this severe heap corruption disappeared by accident in recent versions of dash (and it could be still triggered) or if it was properly fixed.