lsof FD column equal to DEL, what does it mean?
lsof usually reports entries from the Linux /proc/<PID>/maps file with mem in the TYPE column. However, when lsof can't stat(2) a path in the process maps file and the mapsfile entry contains (deleted), indicating the file was deleted after it had been opened, lsof reports the file type as DEL.
Yes, Simply those files are deleted after they are read by the process. If you have updated/replaced those files then you probably want to restart the service/process.
It could be that the process has the file in memory but It has been deleted by any other process?
Yes, or by the same process.