I believe there are some known issues with Excel export in version 19.2 of SQL Developer. A fix should be under way.
Does it work with SQL Developer 19.1?
For that (xls) format, the Excel upper row limit is only 65536, so you are near the limit anyway. If you export to the xlsx format, that should get you to 1M rows, except perhaps the aforementioned known issues in 19.2 may get in the way. Exporting to the csv format is probably the most robust approach.