I was actually thinking about Susan Boyle the other day and how she was all the rage back in 2009-2010 and still even managed to reach #1 with her third album in late 2011.
Although she recorded and released 4 further albums during the rest of the 2010s (plus her last one that's a semi-'greatest hits' release titled 'Ten' in 2019), she seemed to pass largely unnoticed post-2013.
According to Wikipedia, she still remains active overall as a singer/performer, but hasn't actually recorded or released any new material since 2019.
I also remember Frankie Goes To Hollywood, who were mega popular and successful during 1984-early 85 suddenly dying a death when they released their second album in 1986, and they were barely heard of again after. Ditto for Paul Young whose first 2 albums were mega successful back in the mid 80s, was already mostly forgotten about from about 86 onwards when he released his third album.
Can anyone think of any others over time?