Hi, are the missing emails visible when you sign in to your mailbox using a web browser?
Open Outlook on iOS, go to Settings, tap your account, then tap Reset Account to resync all data. If the emails are still missing, remove the account from the app, restart your iPhone, then add the account again using the correct account type. Next, sign in on the web and check if the emails exist there, if they are missing on the web you need to check Focused Inbox, Filters, and Deleted or Archive folders, then recover items if needed. These steps force a full sync and confirm if the emails are stored on the server or not.