SMART Extended self-test routine changes duration! - WD Elements 3TB

I have now purchased 2 WD Elements 3TB which exhibit the same behaviour.

These are both NEW drives. Fresh out of the box.

When I first get the drive, SMART shows this:

Short self-test routine 
recommended polling time: 	 (   2) minutes.
Extended self-test routine
recommended polling time: 	 (   7) minutes.

I then ran a short SMART test and a long SMART test, which comes back ok:

SMART Self-test log structure revision number 1
Num  Test_Description    Status                  Remaining  LifeTime(hours)  LBA_of_first_error
# 1  Extended offline    Completed without error       00%         0         -
# 2  Short offline       Completed without error       00%         0         -

Then, I ran a badblocks test which shows no bad blocks. (It writes, 0xaa, 0x55, 0xff, 0x00 to the disk from beginning to end and tests the result each time.)

Then, I ran another short self test, which also comes back ok:

SMART Self-test log structure revision number 1
Num  Test_Description    Status                  Remaining  LifeTime(hours)  LBA_of_first_error
# 1  Short offline       Completed without error       00%       261         -
# 2  Extended offline    Completed without error       00%         0         -
# 3  Short offline       Completed without error       00%         0         -

Then I ran a long SMART test again, and it gets stuck on 90%, even after 24 hours!

Self-test execution status:      ( 249)	Self-test routine in progress...
					90% of test remaining.

At some point during this, the extended self-test routine time has extended from 7 minutes to 682 minutes!

Short self-test routine 
recommended polling time: 	 (   2) minutes.
Extended self-test routine
recommended polling time: 	 ( 682) minutes.

I have returned one already thinking it was a fault, but now I’m thinking this could be a firmware issue.

Is there perhaps a firmware update to fix these drives?

Cheers,
Tudor.