Could you please share a bit more about your scenario?
Do you want the user to upload attachment to replace the uploaded attachment when he edit the item?
Based on the issue that you mentioned, I have made a test on my side, and the issue is confirmed on my side. Actually, it is an known issue with Attachments field -- when set the MaxAttachments property to 1, when you attached a file to the control, the prompt message would show up as below:
and the "Attach file" option would be disabled. I afraid that there is no way to achieve your needs in PowerApps currently.
If you would like this feature to be added in Attachments control, please submit an idea to PowerApps Ideas Forum: