Volume screen indicator issue on my iPhone

Recently bought to my girl a 14 pro. She noticed that sometimes the volume indicator works alone without touching the volume buttons, and when we decrease or increase the volume by using the buttons the volume return to its initial status by itself! Am attaching a you tube link for the problem:

https://youtube.com/shorts/qSe8hkcRFq0?feature=shared

thank you in advance.

[Re-titled by Moderator]