@Kouta 

Hi,
Typically, the camera won't change the Video mode automatically. We recommend sending an email to support.usa@tp-link.com with the following information for further assistance.

Subject: [Forum Escalation][ID 835936] Tapo C100, video mode keeps changing from day mode to auto mode 
Forum Nickname: [User Provided]
Thread URL:  https://community.tp-link.com/us/smart-home/forum/topic/835936
Model&Version: [User Provided]
Description: [User Provided]
Please provide the following information about the reported case.
  1) The phone's model number and OS version. The Tapo app's version info.
  2) Have you shared your TP-Link account or the camera with others?
  3) Collect the camera log after the video mode changes and send it to the support team.

Once sent, a ticket will be created in our support system, and a member of the team will follow up to gather more information or troubleshoot a cause.

Best Regards