Something changed recently and my signature has become massive, I use size=1 to keep it small and discreet.
Adding the default line-height of 1.618em (i.e. <font size=“1” style=“line-height: 1.618em;”>) solves the issue.
(Actually IIRC it used to be even smaller, e.g. ~1.3em)
EDIT: Checking the signature preview it displays correctly.