No, they are ANDed together, this one is simply a bug that, apparently according to my notes, has been there pretty much forever.
A property group specified for breaks also gets selected for rounds. eg, create a group for 40 seconds or less during a break; it will always be selected when the clock hits 40 seconds, in round or break
Without digging too deep, it seems like maybe this is true if you set the "Breaks" condition to "All". If you set it to a specific break number, seems like it works. So, in quickly testing this, I used the Default screen set for my main tournament screen(s). Then for break 1, I added a new screen set with the conditions "Break" set to "1" and "Seconds left" set to "<= 120". Did the same for break 2, but changed "Break" to "2". Works fine for me.
FYI, I am working on allowing formula-style conditions for screen sets, property sets, etc. Hopefully in the near future...