Starting with 6, Google implemented severe restrictions on the ability of apps to work in the background.
In 7, they took them to the next step, making them more severe.
This is the reason, but it's not an "issue" from their point of view -- to them, this is "making Android even better" (because this, no question, improves battery life).
Aqua Mail asks the user to "whitelist" itself from these "optimizations", but even doing this, does not fully remove the limitations. Android still won't let the app work in the background more often than every 15 minutes.
