Is your Assign Footprints window large enough? Can you maximise it? If the window is too small then the Filtered Footprints column may be truncated.
Although I cannot get the column to totally disappear or become blank, but this is on Linux and may be different on another platform.
Another line of inquiry is the Footprint Filter. At the top of the window are three icons representing the different possible filters: Filter by Filters in Symbol, Filter by Pin Count, and Filter by Library.
These icons are toggles. The result set is the intersection of all the active filters so could be empty. Which ones have you turned on? Click on them to see if the results change.
Also, in the starus area at the bottom of the window, the state of the filters is shown. For example:
What does your status area show?