When working with durations, never (yes, never!) use wall time format like HH:MM
; always use duration formats like [HH]:MM
, which will easily show your problems.
And when working with times, be aware that drag-copy operations on numbers (and times) typically increase numbers by one: test with a cell with value 1, and drag-copy it down (select the cell and use mouse to drag the small dot in the bottom right part): it will turn 2. When a time cell (with something like "19:45", which is actually a number like 0.822916666666667 formatted like a time) is dragged down, it is also incremented by 1. But having formatting in the cell like "HH:MM", which is possibly something you actually need here (if you register some wall time here), you won't see that added 1 (which would be 1 whole day), and only see "19:45" again, without realizing that the value is now much greater.