Why is the number of threads not kept during Yandex Tank load testing?

Question

stress testing

Hello.

I have a virtual machine on the Internet (4CPU, 16 Gb) and a service in inside with 80/443 exposed to the Internet.

decided to find out what maximum load our service can withstand

for these purposes, it was decided to use Yandex Tank

prepared

phantom:
  address: 201.1.5.1:443 #IP тестового стенда, порт 443
  ssl: true #используем протокол https
  uris:
    - "/"
  load_profile:
    load_type: rps
    schedule: const(5000, 240s) #удержание 5000 rps в течение 240 сек
  instances: 5000
  header_http: "1.1"
  headers:
    - "[Host: www.service.ru]"
    - "[Connection: keep-alive]"
    - "[User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36]"
uploader: #модуль для постройки графика
  enabled: true
  operator: my-username
  package: yandextank.plugins.DataUploader
  token_file: token.txt
console:
  enabled: true
telegraf:
  enabled: false

test result here: https://overload.yandex.net/427053

I noticed that Tank scored cruising 5K Treads and then dropped to 100 threads.
while the test profile

schedule: const(5000, 240s) #удержание 5000 rps в течение 240 сек
  instances: 5000

Why can this happen?