SD38.93: CHECKPOINT partner tag BY 1/4 right from 3x1 triangles within a 2x6

[GuySteele]

This works fine, from Z formations within a 2x6—SD correctly realizes that the girls are the checkpointers:

Sd 39.83 : db39.83 : ui1.13tty     C2

 1:   HEADS pair off
 2:   touch 1/4
 3:   head corners truck

  .    3G^   4BV    .    2G^   3BV

 1B^   4GV    .    2B^   1GV    .

 4:   CHECKPOINT partner tag BY 1/4 right

  .    2G<   3G>    .

  .    4B<    .    3B<

 1B>    .    2B>    .

  .    1G<   4G>    .

But this very similar example fails, where the dancers are in 3x1 triangles within a 2x6:

Sd 39.83 : db39.83 : ui1.13tty     C2

 1:   HEADS pair off
 2:   touch 1/4
 3:   head corners truck

  .    3G^   4BV    .    2G^   3BV

 1B^   4GV    .    2B^   1GV    .

 4:   side boys press ahead

  .    3G^    .    2B^   2G^   3BV

 1B^   4GV   4BV    .    1GV    .

Can't do this call:
CHECKPOINT partner tag BY 1/4 right
Can't find checkpoint people in this formation.

I was expecting the result

  .    2G<   3G>    .

  .     .    2B>   3B<

 1B>   4B<    .     .

  .    1G<   4G>    .