A bug on Mongodb 4.2.7 has been identified- and a fix is included in 4.2.8 .Certain conditions are required for the bug to appear - detailed below. Based on the supporting documentation for the MongoDB 4.2.7  bug to occur:

• Must be running MongoDB 4.2.7.
• Managed by Ops Manager 4.2.13+ or managed in Cloud Manager with MongoDB agent version v10.14.0+.
• Incremental backups are enabled.
• The DB server has  collection files greater than 2GB on disk.
• These collections are modified anytime after backups are enabled.

If these conditions are met , there is a possibility that MongoDB will improperly overwrite memory.

Read here for greater detail supplied by MongoDB

To fix you will need to upgrade the Replica Set version from 4.2.7 to 4.2.8 . The upgrade does require a host restart - so ensure all checks are made regarding data recovery points and any existing users connected which which you can get using How to list MongoDB connections - client ip and port