We used MS Graph REST API to create conversations and messages (over 20000 messages) in Team's private channel at 14 Jan 2021 at once, and then create a few conversations and messages to the same channel again at 18 Jan 2021. All the conversations and messages can be loaded successfully at Teams desktop application (Win10) and Teams on the web, but cannot load the conversations and messages by Teams Android app and iPhone app.
The API we used as below:
Create channel message: https://graph.microsoft.com/beta/teams/{0}/channels/{1}/messages
Reply message: https://graph.microsoft.com/beta/teams/{0}/channels/{1}/messages/{2}/replies