When starting mongocryptd, I see the following message printed:

2020-01-30T18:09:35.739-0500 I  CONTROL  [initandlisten] Using lock file: /home/w/apps/ruby-driver/mongocryptd.pid

The lock file and the pid file in Unix are different concepts.

Lock file: https://dmorgan.info/posts/linux-lock-files/
Pid file: https://stackoverflow.com/questions/8296170/what-is-a-pid-file-and-what-does-it-contain

My reading of the source code (the createLockFile function) suggests the file should be named a "pid file" based on how it is used (I do not see any locking being done with it).

This function references the same path as a "pid file" in other messages.

Tested on 4.2 server built from source recently.