Here is what is going on.
1) trigger value switched to "Problem" at 2015-12-18 08:57:21, generating an event (ID = 734038)
2) action sent an email as expected
3) maintenance period was activated for that host (after the problem started)
What I have noticed is :
A) escalation step continues to increment and emails to be sent, and yet there is a condition "maintenance status is not in maintenance". The condition is ignored, probably because the action "loop" started before the host was set to maintenance. That behaviour is odd.
B) a change in the step duration is effective only after the initial step duration is elapsed. ie. If I switch the step duration from 3600 seconds to 60 seconds, it takes 1 hour before the step duration switches to 60 seconds...
C) the step increment stops if I disable the host and enable it again after a few minutes. In other words, the strange behaviour described in (A) is fixed by disabling / enabling the host : the action's condition "maintenance" is now effective