Hi @IBN ,
Welcome to our forum!
Looks like your issue is more related to Outlook for web, please kindly understand that the Outlook tag here we mainly focus on general issues about Outlook desktop client.
Based on my research on this issue, I found a thread similar to your issue, which mentioned that it may be the setting of some rules that led to this situation, and is consistent with Nathan-P's idea. It is recommended that you check whether your rule is related to the Conversation History folder.
If nothing is listed there and you are using an O365 account, please refer to the suggestions of Rick Song MSFT in this thread to contact the Office 365 admin and use the PowerShell to get the Inbox Rules.
If you are using an Outlook.com account, in order to better solve your problem, it's suggested that you could post your case in dedicated forum for Outlook for web.
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