[1][2] Swimming the first leg of the 4×50 meter mixed freestyle relay at the 2021 FINA Swimming World Cup stop held in Berlin, Germany, Diehl contributed to a gold medal-win in a world junior record time of 1:31.50 with finals relay teammates Quintin McCarty, Kristina Paegle, and Carly Novelline.

[4][5] Six days later, at the stop held at Danube Arena in Budapest, Hungary, he helped win the gold medal in the 4×50 meter mixed freestyle relay again, this time splitting a 22.45 to contribute to the final mark of 1:32.34.

[2][10] Commencing competition on day one at Veterans Memorial Aquatic Center for the 2022 Junior Pan Pacific Swimming Championships, held in Honolulu in August, Diehl set a new Championships record with a personal best time of 53.40 seconds in the morning preliminaries of the 100 meter backstroke and qualified for the final ranking first.

[13][14][15] Later in the finals session, he led-off the 4×100 meter mixed medley relay with a 53.42 to help win the gold medal in a new Championships record time of 3:46.83.

[16] Later in the session he led-off the 4×100 meter medley relay with a 54.24, helping achieve a Championships record time of 3:36.65 and win the gold medal.

[30] On day two of the 2022 U.S. Open Swimming Championships, December 1, Diehl swam a personal best time of 2:00.50 in the preliminary heats of the 200 meter individual medley and qualified for the final ranking second.

[38] For the evening final, his last race of the Championships, he swam 1.21 seconds faster than in the morning, winning the gold medal with a personal best time of 1:56.41.

[39] The following week, Diehl placed second in the 200 yard individual medley at the 2022 Winter Junior US National Championships edition East with a personal best time of 1:43.01.