Not new. Had this occur on 13.5 and 13.5.1 sometimes. Cannot really see a pattern when that happens. Just yesterday I dropped below 20% while using my iPhone 11 with 13.6b3 and I just got the notification once.
Noticed this Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_4) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/13.1.2 Safari/605.1.15 as the Ua In Safari.