During a DB patching I found this note in the patch README file.
This patch is non-Data Guard Standby-First Installable -
Please read My Oracle Support Note 1265700.1
This is the first time I see this, so I was curious about this as Oracle recommends to apply the patches first to the standby database and then patch the primary database.
After reading the note id 1265700.1, found that we must shutdown both the primary and the standby and apply the patches at the same time.
This worked well and both databases were patched.