SMART Extended self-test routine changes duration

Question

I purchased a new Western Digital Elements 3TB a couple of weeks ago.

As usual, I did all SMART tests, then badblocks, then SMART tests again.

When I first got the drive, SMART showed 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 came 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 full badblocks destructive write test which showed no bad blocks.

Then, I ran another SMART short self-test, which also came 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 changed 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 returned it thinking it was just a fault with the drive, but the replacement (also new) shows exactly the same error after the same sequence.

I have posted this issue to the WD Community but got no response.

Is this a batch issue (like a faulty EEPROM command) or a known issue (e.g. SMART firmware overrun tripped by a long sequence of 0xff?) with the drive? Should I return and try again or change to a different make/model?