It continuously requests and sends status 200 responses.
# nginx access log
GET /api/v4/system/ping?time=xxx HTTP/1.1][Mattermost/364 CFNetwork/1126 Darwin/19.5.0
POST /api/v4/users/status/ids HTTP/1.1] [Mozilla/5.0 (iPhone; CPU iPhone OS
some error log
# nginx error log
upstream prematurely closed connection while reading response header from upstream, client: x.x.x.x, server: domain.tld, request: "POST /api/v4/users/status/ids HTTP/2.0", upstream: "http://x.x.x.x:8065/api/v4/users/status/ids"