I am unsure of how a trade offer can change automatically and get accepted on your phone with out your knowledge!
Do you have any chat logs from this said user? If you do please provide
Uncropped and Unedited screenshots of your complete chat with the user from this page https://help.steampowered.com/en/accountdata/GetFriendMessagesLog
Please find them on a web browser and hover your mouse over the username of the account in at least one of the pictures, so we can see the URL at the bottom left corner of the screen.
Example- should look like this https://imgur.com/a/ImcDuhP
- Screenshot of the trade offer - should also be Uncropped and Unedited, also in a web browse, with mouse your hovering over the username so that profile URL pops up in bottom corner.
- Screenshot of the trade offer - with mouse hovering over the item in question