I was working at a customer database environment, setting up nearly 30 new databases with oracle 12.1 and 12.2 Standard Edition and Enterprise Edition version. For the datafiles we used ASM with 12.2 Grid Infrastructure for Single Instances (with Restart) and the newest version we got(April 2018).
One of the things I was really going mad was, that some entries at oratab were gone after stopping and/or starting the database with srvctl or by rebooting the nodes. .
First I was thinking about a bug at ASM, because there was also one at 12.1.
But after a while I found a nice note at https://support.oracle.com which states, that this is a new default behavior and not a bug.
In detail, starting with 12.2.0.1.171017 GI RU/PSU (or any version later), the oratab entries are removed automatically, if they have been "added by agent".
The note "Applying 12.2.0.1.171017 RU (patch 26737266) To 12.2 Cluster Removes Oratab Entries (Doc ID 2329359.1)" tells us, that "The agent deletes the oratab entries that have comment "# line added by Agent". For example, when the GI is recycled (including after a node reboot), then the agent will remove the oratab entries that have the comment "# line added by Agent"".
Unfortunately, it was not enough to just remove the comment "# line added by Agent", like it is written at the document - we needed to put own comments "# manually added" at the end of the lines.
Why is Oracle doing this as new default behavior? "With the introduction of the flex ASM (asm may not run on all nodes) and policy managed databases (no instances is tied to a server), the mapping of the asm and RAC database instances to a specific node is no longer guaranteed to be valid, so this is the reason that the oratab entries are being removed."
So if you run into this problem with GI version 12.2 or later (I think this will also happen with Oracle Database Grid Infrastructure 18c) and you want to keep the entries, just add your own comment at the end of every line you want to keep.
If you don't know it, you are going mad, if you know it - life can be easy.
P.S. If you need to find the names of the databases, this blog entry could be helpful: https://fritshoogland.wordpress.com/tag/recreate-oratab/
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