I ran into roughly the same thing.  I sent in a PR on 13-June as 
a placeholder.  It's kern/10358 


I'm pretty sure it depends upon which chip/board you are using. 

From the PR:

  The DE500-BA has a 21143.  I can't get it to work in full duplex 
  on either Alpha or i386.  (Half duplex works as expected.) 

  The DE500-AA has a 21140A and DP83840.  It works correctly, both 
  full and half.  (Only tested on Alpha.) 

  The motherboards on Miatas and XP1000s also have 21143s.  They 
  work correctly both full and half.