Ancheer electric mountain bike cuts out under load
Your battery pack is failing under load and your battery management system is shutting off to prevent your pack from going up in flames. 

A voltage drop like this is often a sign of one or more cells in your pack has gone bad - whether due to age, too many charge cycles, improper battery management, or a manufacturing defect. Or the pack could have simply been sized, engineered or manufactured without enough high current capacity for your motor’s maximum current needs. 

You could also try to eek more life out of the pack by dropping the assist level when going up hills. Cells often fail when under severe load like this. Cheap ebikes are occasionally cheap because the battery packs are made by recycling old laptop cells, so your pack may have arrived already pre-aged. 

If you’ve had it for a year and have ridden it every day, it may just be the natural end of life for the pack. Getting a new pack from the vendor may solve some of your problems. Your bike isn’t that powerful (250w nominal) and the hill you describe is quite steep (10 degrees) so even in the best of circumstances you might have to pedal hard while turning down the assist level.