Is there a different voice actress singing the same ending for each arc?
Yes.
From Wikipedia,

The ending theme is &quot;Fukashigi no Karte&quot; (不可思議のカルテ), with each arc using versions by Asami Seto (Mai Sakurajima), Yurika Kubo (Kaede Azusagawa), Nao Tōyama (Tomoe Koga), Atsumi Tanezaki (Rio Futaba), Maaya Uchida (Nodoka Toyohama), and Inori Minase (Shoko Makinohara) under their character names.