Since the beginning, Uptime Robot was supporting URLs with SSL certificates (https).
However, self-signed certificates (the ones with unknown CAs) were not supported as they were generating errors like "unknown certificate", etc. and Uptime Robot was considering these websites to be down.
Today, URLs with self-signed certificates are now supported just like any other URL (as you already know the certificate is self-signed and no need to throw alerts).
On the other hand, for any type of https URLs, if the certificate is an invalid one (like the common name and URL not matching) or the date is expired, Uptime Robot will keep sending a down alert.