I had this issue and realized I had forgotten to set my model to IEnumerable after upgrading my project to GlassMapper 5+ with Sitecore 10.1
Edit attempt 3:
It would be difficult to update placeholder text in the input field because whenever you will update placeholder text, the cursor will show in the input field.
There could be one way where you can add Edit Frame in between input fields like below:
@using (BeginEditFrame(Model.Page, "Edit PlaceholderText", x => x.PlaceholderText))