It is very important to read the documentation before to use the Oracle facility of Flashback Database. Furthermore, you have to take a full rman database backup to be protected against any contingency (bugs or restrictions) of flashback database features.
If you create restore points and after that you do a datafile resizing, then you have to create again a new restore point, because you cannot flashback database across a datafile resize. For example:
SQL> CREATE RESTORE POINT before_test0 GUARANTEE FLASHBACK DATABASE;
SQL> ALTER DATABASE DATAFILE 6 RESIZE 2048G;
SQL> CREATE RESTORE POINT before_test1 GUARANTEE FLASHBACK DATABASE;
For more information, please read the oracle documentation.