I recently acquired a 2.18m wingspan carbon fiber VTOL aircraft from a local manufacturer. The aircraft is a carbon fiber version of the HERO VTOL 4+1.
The empty weight of the aircraft without battery is 5,225g. With battery the aircraft weighs 7,655g. The battery in use is a Tattu 22000mah 6s.
I installed a camera and a video data telemetry device in the drone and went for my first flight test. I first tested the drone in QSTABILIZED mode to be sure the controls all worked fine. I did a few hops and it was good. I then planned a short way point mission around my flying area. The mission involved a VTOL TAKEOFF, about 4 waypiont aroun my flying area, and a RTL. The plane tookoff very fine and transitioned nicely. It returned back and transitioned back to QUAD mode above the home position where it took off form. The descent started and got faster until it crashed hard into the ground. I tested the drone of few times again and got varrying results. There was a time it completed a similar short mission and landed successfully. I then repeated the same successful mission immediately after and it crashed on landing because it decended too fast. I have gone through the logs as best as I can cause I am not really an expert at it. I saw that the battery voltage drops too fast and then returns back to normal after landing or crashlanding. This made me try two other batteries that are known to be in good condition. First time of testing with the new batteries it seems to work fine but then begins to feel underpowered. There were times when the aircraft would not complete the AUTO VTOL TAKE OFF and begin to descent rapidly into a crash.
Below is a link to the Tlog and .Bin file.
https://drive.google.com/drive/folders/1oipcymrj7cL3ZsVhGNKABiZdNweiax3K?usp=sharing
Pls, help me detect the issue. 
Thanks in advance.