Hosts in maintenance do not affect SLA calculation normally as expected, but if a host has been in maintenance for an extended period of time, it will eventually start to affect SLA calculation negatively again. Unfortunately i don't have exact number at which point this starts as i've noticed this in various phases, but i have following:

Service with SLA that has a monthly reporting period. I've seen it now multiple times that matching hosts that have been in maintenance for at least a month (in some cases 2-4months) suddenly start to affect the calculation negatively while still being in the maintenance. These hosts were part of a maintenance which has a very long activity period so it hasn't for sure expired in the middle either. This feels as if there was a mechanism that evaluates the maintenance from only certain days in the past.