Just to be sure:

With saving you mean if you reload the SolidCP page --> the signature is gone?

or is it still there it's just never passed to Smartermail?

As in my dev lab where i tested SM 16 (and fixed some bugs) i did set signature, reloaded the page and it was still present. i did not verify in SM gui if it was set in SM it self (as i didn't think that was the reported issue at hand).