If the grade is overriden in the gradebook - then one wil not be able to change it from the activity (assignment) anymore.
Try going to your "PPT" assignment on the course homepage, and update grade from there. The interface will be greyed out and not available to you.
The grade dissappeared most probably because you have not used assignment interface at all from the beginning
Thus, you overridden your non-existed/empty grades. No harm done, but this is why the highlighted column.
Gradebook
Highlighted Columns in Gradebook
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