dbmethods:auth(username, password) throws error

[sunao-uehara]

Currently I am having really weird issue around db authentication that dbmethods:auth(username, password) throws following error. But it happens randomly, not all the time. I would say it is like 10- 50%

May 13 17:21:03 xxx/xxx: [error] xxx/xxx/resty-mongol/dbmt.lua:75: no such cmd: user

So I checked the line, it should execute mongodb command "authenticate". but actually it tried "user" somehow, then throws error as it doesn't exist.
Even weirder thing is that the issue happens more frequently on high spec linux machine than low spec one...

Has anybody seen this issue?