I created a new entry at 12:38, and used the “Move start time back” option to start one hour back 11:38. Then I tried to adjust the start time even further. First I selected the hour, it turned blue, I changed the time even further back 11:00, and clicked elsewhere to accept. Not only the change did not stick but after a while, it changed to 12:45, the actual hour at that time.
Thank you for the detailed bug report, Tomas! I have fixed this now in code and will release an update soon.