Are the other instances also running Ruby 2.3.1-2~ubuntu16.04.14 ?
maybe nothing relevant but:
so this was clearly a Ruby bug. We tested across multiple versions of Ruby and determined that only rubies 2.3.x and 2.4.x were exhibiting the leak (apparently this was fixed in Ruby 2.5.0****).
And the discourse readme asks for [Ruby 2.6+] 