Mail Flag Colors Between iOS & iPadOS

1. Is this your corporate iPhone? Then please contact the system administrator of your corporate
2. If it is your personal device, in that case, do you have any profile installed by your corporate/organisation/enterprise/School Management?
   1. Erase the profile added by you If you have installed a profile on your own --> Install or remove configuration profiles on iPhone - Apple ...
   2. Or please contact the system administrator of the organisation who provided the profile to you

This Apple article may give you some clue --> Flag emails on iPhone – Apple Support (UK)

If your incoming email messages are incorrectly flagged, the primary cause could be misconfigured rule / filter, or a setting on your Forwarding and POP/IMAP tab. To resolve this issue, please make sure you haven't created any rule / filters with the action to Delete / Move it that would affect the email messages being moved automatically to another folder.

Please log into the email account using a web browser on PC/Mac and check the settings.

Which email account that you are trying to refer here? Google, iCloud, AOL, Exchange, Outlook, Yahoo or any Other