Why is my PDB seemingly stuck in RESTRICTED mode?

Check out the view pdb_plug_in_violations - it will point you in the right direction.

In my case, I had created a pluggable database some days after my CDB was finished.

I had completely forgotten about a common user that I had created at that time:

-- create a common user for all CONTAINERS
-- common users must use the prefix c##
create user C##dba identified by MySecretPasswor
default tablespace USERS
temporary tablespace TEMP
quota unlimited on USERS
container=all;

The new PDB was created without the USERS tablespace, which prevented the new PDB to be synchronized with the parent container.
The pdb_plug_in_violations contained the following message:

Sync PDB failed with ORA-959 during 'create user C##dba identified by *default tablespace USERS
temporary tablespace TEMP
quota unlimited on USERS
container=all'

To resolve the situation, connect to the container with the missing tablespace:

alter session set container=pdbveg2;

Make sure my session OMF parameter is correctly set:

select name,value,DEFAULT_VALUE,ISDEFAULT,ISPDB_MODIFIABLE, ISSES_MODIFIABLE, DESCRIPTION
from V$PARAMETER where name like '%create_file_dest%';

NAME	VALUE	DEFAULT_VALUE	ISDEFAULT	ISPDB_MODIFIABLE	ISSES_MODIFIABLE	DESCRIPTION
db_create_file_dest	/u02/oradata/cdbveg/pdbveg2	NONE	TRUE	TRUE	TRUE	default database location

Create the missing tablespace:

create tablespace USERS
datafile size 8M autoextend on next 2M maxsize 2G;

Finally, close and reopen your pluggable database:

alter pluggable database pdbveg2 close;
alter pluggable database pdbveg2 open read write;

Check status:

select CON_ID,name,OPEN_MODE, RESTRICTED
from v$containers;

CON_ID	NAME	OPEN_MODE	RESTRICTED
4	PDBVEG2	READ WRITE	NO