When there are multiple Microsoft Exchange calendars (from different accounts) with the same name (Calendar) and there are multiple lists (smart or reminder) using exchange calendars with the same name, albeit different calendars, Goodtask seems to get confused.
GoodTask is constantly confusing which calendar to use. I will select a specific calendar to use for a list, but then later on, on the same or different device (it happens on both OSX and iOS) it seems to "forget" what calendar is was suppose to use for the specific list and instead uses the other calendar with the same name.
Although I cannot pinpoint the problem, my assumption is that it has to do with the order the accounts are listed in the Accounts preference panel, or the list of calendars within GoodTask itself. Or, perhaps GoodTask is simply using the names of the calendars as an identifier rather than a more unique identifier.
Unfortunately I cannot change the names of the Exchange calendars as this isn't allowed by the provide. I come across no issues with the calendars that are part of Google accounts.