It could possibly be the internal change in size to some columns in the data dictionary.
eg
SQL> desc user_objects
Name Null? Type
----------------------------- -------- --------------------
OBJECT_NAME VARCHAR2(128)
SUBOBJECT_NAME VARCHAR2(128)
OBJECT_ID NUMBER
DATA_OBJECT_ID NUMBER
OBJECT_TYPE VARCHAR2(23)
CREATED DATE
LAST_DDL_TIME DATE
TIMESTAMP VARCHAR2(19)
...
Notice that many dictionary names have changed from 30 bytes to 128. In 12.1, the limit is still 30, but the *dictionary* has changed to 128 (which was then made "public" in 12.2).