Attach sig default in PM off, but global default on?

[Nobody Special]

I just noticed this: When I send a PM, the box for Attach a signature is unchecked. In my Board default settings, I have it set to always Attach by default, and it usually has (on both forum posts and PMs), until now. I recently turned off Multiquote, and, thinking it might have been the culprit, turned it back on. But no, it still won't auto-attach my sig in PMs.

Help?

[Nobody Special]

Oddly enough, when I try to send PMs to other members, the box is checked. I'm asking the one individual whether they have signature display turned off; that may be the issue.

[Nobody Special]

The person in question has attachment of sigs disabled, due to generally responding from a nonstandard device. This may be the culprit.

I'll assume that since I've gotten no reply to this, that it's either a known thing or too minuscule to appear on anyone's radar.

(The cloak-and-dagger stuff about keeping the person anonymous involves an ongoing game, so.)