Mongodb – Why MongoDB collections are dropped automatically

I have installed mongodb 3.6 standalone instance and created some collections through insert query in mongo shell. My app points to the db server, the app creates some new collections, it reads, writes and so on. This is running for more than two months. If we restart the instance we can see in mongodb collections which are created through insert query are lost. Now we can see the same behavior in other environments. Also, while analyzing this problem, we found that during mongodb server start there was a error saying "mongodb failed to create WiredTiger bulk cursor: Device or resource busy". Any idea what that error line implies?

The below messges are found in mongodb log file during server shutdown

**2018-08-28T12:44:46.138+0530 I CONTROL [signalProcessingThread] got signal 1 (Hangup), will terminate after current cmd ends**

**2018-08-28T12:44:46.138+0530 I NETWORK [signalProcessingThread] shutdown: going to close listening sockets...**

**2018-08-28T12:44:46.138+0530 I NETWORK [signalProcessingThread] removing socket file: /tmp/mongodb-27017.sock**

**2018-08-28T12:44:46.152+0530 I FTDC [signalProcessingThread] Shutting down full-time diagnostic data capture**

**2018-08-28T12:44:46.158+0530 I STORAGE [signalProcessingThread] WiredTigerKVEngine shutting down**

**2018-08-28T12:44:46.265+0530 I STORAGE [signalProcessingThread] shutdown: removing fs lock...**

**2018-08-28T12:44:46.265+0530 I CONTROL [signalProcessingThread] now exiting**

**2018-08-28T12:44:46.266+0530 I CONTROL [signalProcessingThread] shutting down with code:0**

And during server startup the below messages are found in the log file,

**2018-08-28T14:19:39.256+0530 I STORAGE [thread2] createCollection: config.system.sessions with generated UUID: e981a966-4f87-45a9-9adb-6b7e2fdb8803**

**2018-08-28T14:19:39.277+0530 I INDEX [thread2] build index on: config.system.sessions properties: { v: 2, key: { lastUse: 1 }, name: "lsidTTLIndex", ns: "config.system.sessions", expireAfterSeconds: 1800 }**

**2018-08-28T14:19:39.277+0530 I INDEX [thread2] building index using bulk method; build may temporarily use up to 500 megabytes of RAM**

**2018-08-28T14:19:39.278+0530 W STORAGE [thread2] failed to create WiredTiger bulk cursor: Device or resource busy**

**2018-08-28T14:19:39.278+0530 W STORAGE [thread2] falling back to non-bulk cursor for index table:index-12--7471022903774062594**

**2018-08-28T14:19:39.278+0530 I INDEX [thread2] build index done. scanned 0 total records. 0 secs**