Was fixed via:
https://github.com/discourse/discourse/commit/3d76fb9c2c400fb91d5f77247e9b776ea5c8912a
I am not sure if this classifies as bug. It feels odd to me to clear the date range field. Why not select the default date range (a month) instead of manually clearing the date field?
Many times I run the report with only “Start Date” set assuming that the end date will automatically be considered as today’s date, but I can understand that it will be more clear/explicit for end user if we automatically fill “End Date” with today’s date. Same applies for start date.
Added on my list. 
EDIT: I just realized that this bug report is over 8 months old and I edited my response accordingly.