The sound "mob/panda/worried/worried1" never plays; only worried2 through worried6 play. worried1 is not mentioned in the sound list for the event entity.panda.worried_ambient, for some reason; however, it does exist as a file referenced the asset index.
(Additionally, "mob/panda/pant2" is one of the sounds that can be played for that event despite its name not matching, but that doesn't necessarily seem like a bug)

To reproduce, find a worried panda and listen or repeatedly run /playsound entity.panda.worried_ambient ambient @s.