Turns out bug #58206 was already open to address this.  Further discussion should be redirected there.

As discussed on the email list (http://lists.gnu.org/archive/html/groff/2021-09/msg00046.html), tmac/pdfpic.tmac contains an external (.sy) call to a shell pipeline that includes a grep command which fails in cases where it falsely detects its input stream to be binary.  Further down the thread, a sed and an awk command are both proposed as more robust replacements for the grep.

Addressing bug #55107 will obviate the need for this shell pipeline at all.  This is the preferred fix for this problem.  But it's a more substantial change, and the fix proposed in the aforementioned email thread is simple and low-risk, so it's something that could be done until 55107 is addressed.