I have imported a GEDCOM created by Legacy 7.5 into Gramps 3 and 4 with the same results of not getting all the people of a surname to list in the detail descendant report.  The number of people listed in the summary of loading the database is correct some of the text reports containing an index of people are correct.  None of the searches I have done on the web has given me a clue as to what the trouble might  or the fix for it.  I did find in one location that their was problems with importing Legacy GEDCOM into Gramps but no details were provided.
Thanks for any help as I am really stuck and can not generate valid reports from a 481 member data base.
May God Bless Your Day!
Howard W Reagan