I have the same problem on a freshly installed Kmail/identity. The HTML signature seems just fine in the identity’s configuration page, but when sending an email using it, most of the style is gone and table borders are HTML defaults (wide, grey, ugly).
I found traces of this problem dating years back, here for example : forums.opensuse.org/t/how-to-correctly-style-an-email-signature-in-kmail/133755
Is there something I’m missing for this to work as intended, or is this really a long-standing bug ?