To reproduce:
1) set radio button Different background for each monitor
2) Set "minutes to change" for one monitor (doesn't matter which) to 1 char long, eg 822 - by typing number, not clicking arrows
3) Click next monitor - minutes tochange is copied across - this is a bug (or doesn't happen if you use arrows to enter the number)
4) Click previous monitor - minutes to change has lost the last digit (eg if 822 was entered it becomes 82)