Yep, I've had a report or two about this phone in the Russian section of the forum.
Same story every time, going by past experiences:
- a manufacturer decides to implement some sort of "battery optimization" (see: Huawei, Xiaomi)
- they know this will break apps, so they whitelist some of the most popular ones (Gmail, G+, Facebook...) so they're not affected
- all other apps that may be affected are broken by this change (made by the manufacturer)
- but hey, the phone and it's so *thin*, so beautiful, and they already got your money, thank you Mr. Customer.
Hopefully their support can tell you how to access their "battery optimizer" somewhere in the phone's Settings and whitelist AquaMail there.