Hi @TrulsB ,
Could you please share a bit more about the issue?
Do you custom a form for your SP List using PowerApps?
Do you mean that the Attachments control/field would have a default attachment when you add a new file?
Based on the issue that you mentioned, I have made a test on my side, and don't have the issue that you mentioned. The screenshot as below:

According to the issue that you mentioned, it seems that the Attachments control does not reset when you add a new item. I have made a test on my side, please consider take a try with the following workaround:
Set the OnNew property of the SharePointIntegration control to following:
Refresh('YourSPList'); /* <-- Add this formula */
Set(IsReset,false);Set(IsReset, true); /* <-- Add this formula */
NewForm(SharePointForm1)
Set the OnEdit property of the SharePointIntegration control to following:
Refresh('YourSPList'); /* <-- Add this formula */
Set(IsReset,false);Set(IsReset, true); /* <-- Add this formula */
EditForm(SharePointForm1)
Set the OnView property of the SharePointIntegration control to following:
Refresh('YourSPList'); /* <-- Add this formula */
Set(IsReset,false);Set(IsReset, true); /* <-- Add this formula */
ViewForm(SharePointForm1)
Set the Reset property of the Attachments control within the Attachments field data card to following:
IsReset

Please take a try with above solution, then re-publish your custom form to your SP List, check if the issue is solved.
Best regards,