it's nothing about the package - "SQL Worksheet" and "PL/SQL Editor" are different tools.
Both of them can compile PL/SQL - in "SQL Worksheet" you can use "Run Statement" to compile the package specification or body, or the "Run Script" to compile both of them(don't forget to add the "/" at the end in "SQL Worksheet") .
Thanks Günter. Especially, the second sentence confirms my assumption, although I would have liked to understand the different behaviour when the files are opened (that is one is opened in SQL Worksheet while the other in Editor).
I'm curious if you have this same behavior in a much more recent version of SQLDev. SQLDev 18 was just released and is the latest/greatest.