If you are using the 'follow the leader' party formation (The one highlighted below), and have your thief in the leader slot then it is impossible to detect traps and move at the same time (When your whole party is selected).
As soon as select your whole group to move, detect traps automatically turns off (My Party A.I is turned off, so that's not the problem). However if you move your thief into another party slot, detect traps works as intended and does not disable when you try to move as a group.
Also I've tried the other party formations, and there is no issue with any of them whether your thief is set to party leader or not, and detect traps works fine, it only fails to work in if your thief is set to leader in the above party formation.
Normally I scout my thief ahead in heavily trapped areas to avoid other party members inadvertently setting them off, but I figured this formation would make a great detect trap formation, as I could put my Thief at the front and not have to worry about other party members overtaking him and accidentally triggering traps, unfortunately it doesn't work