unbound server close TLS request connection immediately, and log "tcp took too long, dropped"

[qyb via Unbound-users]

I've setup 2 unbound node. Server A forward some domain to Server B, and
Server B forward all requests to 1.1.1.1/1.0.0.1. All connection use TLS.

Sometimes, Server B close request from A after first ssl-data
exchange immediately. (I confirm it by tcpdump), and log "tcp took too
long, dropped",  I must restart unbound process under this condition.

I checked the log message and found it comes from "util/netevent.c". May I
need to upgrade libevent?? It's a libevent bug?

Server A: unbunt 16.04, unbount 1.8.1,
openssl-1.1.1, libevent-dev 2.0.21-stable-2ubuntu0.16.04.1
Server B: ubuntu 18.04, unbound 1.8.1,
openssl-1.1.1, libevent-dev 2.1.8-stable-4build1