nginx reverse proxy VS f5 big ip

Hello and Sorry if i'm posting wrong place but i have some problems between NGINX and F5 and please help . . .

So, my deployment is: client>>nginxRP>>>f5>>>coreAPI, i need to stay NGINX front of f5 because they are regionally different place and i need NGINX to cache same requests for better user experience but in this scenario when we performe stress tests 100 or 1000rps , 10-15% of requests are failed with response code 502 bad gateway. all traffic from client to api are protected via https.

we tried two more scenario with same configuration of f5 and NGINX we removed each Consistently:

1. client>>>nginx>>coreapi ---- 0% failed.
2. client>>>f5>>coreapi ----0% failed.

also when we turning off encryption on f5 all working fine,

From this situation i think problem is some how encryption but can't understand if problem is ssl communication betwen NGINX and F5 why only 15% are failed and not 100?

many thanks and i can add more details if you need.