While testing in AIX and HP-UX machines, We got an issue where <server_name>.url and <server_name>.pid files get deleted if node manager is accidently getting re-started again.
If this is the same issue with your system also then please follow below method to fix it.
Open the nodemanager.properties file and add/update following and bounce the servers to check
A value of true causes native libraries for the operating system to be used.
For UNIX systems other than Solaris, HP-UX, or Linux, set this property to false to run Node Manager in non-native mode.
This will cause Node Manager to use the start script specified by the StartScriptEnabled property to start Managed Servers.