This is because a "work unit" cannot have more than 23h 59min per day. So when you let a timer run over the weekend, the work unit for Saturday is 23h 59min long, even though the timer actually ran 24h - so 1 min more. And to make up for that, this minute is then added to the end time - because I consider the duration of the work unit to be more important than the start/end time.
So the question is why can a work unit not be 24h long? The reason is mostly UI: The time picker controls on Android, macOS or iOS don't allow setting a duration/time larger than 23:59. I would essentially have to design my own user time picker control, which is what I did on Windows because the "normal" one was really bad for Desktop usage and it was comparably easy to achieve. I'm not sure if it would be worth the effort to do the same work on the other platforms just to fix edge cases like the one you reported. Generally, WorkingHours is for tracking working hours, not off-time
