This bug is only valid in Gradebooks with more than 20 students. To replicate, make sure you have chosen the default "Show 20" and then:
1-- click on a Gradebook entry
2-- update/revise the grade entries with invalid grades (e.g. 98.456)
3-- click the > button to advance to the next 20 students
4-- you will receive an error saying "There were problems in your last submission. Please see below for details." The names remain the first 20 names and everything appears as you'd expect
5-- fix the entries to valid grades (e.g. 98.46)
6-- hit Save
The page will refresh and show the names of students 21-40 and the grades you've just revised for students 1-20. Therefore, the revised grades will save for students 1-20 as well as students 21-40, and the refreshed page after clicking Save will be 21-40.