Do you have multiple accounts? The signature setting is per account.

I've read several comments that Signature Switch add-on is the culprit that disables the above-then-below positioning feature, for technical reasons. Unfortunately, I didn't find any reasoning about why that's okay without Signature Switch installed, given the technical arguments.

Inside Signature Switch there's a switch buried deep in the mines of Moria to enable the feature, with some funny warnings before proceeding. I appreciate the fear mongering but it can be toggled off with a click so I'm testing it now; the above-then-below behaviour works as hoped, but the proof is whether recipients complain or it breaks something else like scripting. In my experience, doomsayers are usually triggered by large enterprise like Microsoft Server so I'm guessing that's who gets borked processing signatures. I have no idea what function uses signature code identification, except perhaps filtering on client or server.

Look for the switch as the nethermost item of Signature Switch Preferences/Miscellaneous/Placement/Place signature above reply-quote / forwarded message not recommended

I responded to an email today that definitely came from an Exchange service, after throwing my aforementioned switch, and my sig appeared at the top of my edit, the topmost position. At least it was closer to my edit for cut and paste.