Based on a bit of work I did last night with a safety relay, I think the state changed as you release the GUI button. If an external estop was enabled at that time, the GUI button did not unpress.
I don't think anything with estop is going to wait around for a few seconds. Its pretty much instantaneous. Get it ready before you take it out of estop or nothing will happen.
I kind of ran out of time last night. I got the safety relay engaging on a GUI estop press but I can't yet see a way to turn the machine off from the GUI. I suspect when I look at my press brake control behaviour, this is pretty normal. On It, I either have to turn off the physical power switch or press the estop to turn off the hydraulic pump (which is in the estop circuit). I'm still thinking about that!