process.stdout.write doesn't log in same line (with random line breaks)

What's going wrong?

When using process.stdout.write - output not in the same line. Also the line breaks are random.

How could we reproduce this issue?

// index.js
process.stdout.write('this');
process.stdout.write('should');
process.stdout.write('be');
process.stdout.write('in');
process.stdout.write('one');
process.stdout.write('line');

pm2 start index.js

Output for pm2 logs:

0|index  | this
0|index  | should
0|index  | beinoneline
0|index  | this
0|index  | should
0|index  | beinoneline
0|index  | thisshouldbeinoneline
0|index  | this
0|index  | should
0|index  | bein
0|index  | one
0|index  | line
0|index  | this
0|index  | should
0|index  | beinoneline
0|index  | this
0|index  | shouldbeinone

Supporting information

$ pm2 report

===============================================================================
--- PM2 REPORT (Thu Nov 15 2018 14:01:23 GMT+0200 (Israel Standard Time)) -----
===============================================================================
--- Daemon -------------------------------------------------
pm2d version         : 3.2.2
node version         : 11.0.0
node path            : /usr/local/bin/pm2
argv                 : /usr/local/Cellar/node/11.0.0/bin/node,/usr/local/lib/node_modules/pm2/lib/Daemon.js
argv0                : node
user                 : sasha
uid                  : 501
gid                  : 20
uptime               : 4min
===============================================================================
--- CLI ----------------------------------------------------
local pm2            : 3.2.2
node version         : 11.0.0
node path            : /usr/local/bin/pm2
argv                 : /usr/local/Cellar/node/11.0.0/bin/node,/usr/local/bin/pm2,report
argv0                : node
user                 : sasha
uid                  : 501
gid                  : 20
===============================================================================
--- System info --------------------------------------------
arch                 : x64
platform             : darwin
type                 : Darwin
cpus                 : Intel(R) Core(TM) i5-6267U CPU @ 2.90GHz
cpus nb              : 4
freemem              : 2403622912
totalmem             : 17179869184
home                 : /Users/sasha
===============================================================================

Start with the index.js reproduction and run pm2 start index.js followed by pm2 logs to observe how successive process.stdout.write calls are grouped. Trace PM2's log-stream handling to determine why writes receive random line breaks; done means the six writes appear in one line in pm2 logs.