This change seems to have come in as part of the latest update to the ISTD books when they moved from the single combined revised technique to the five separate books.
The older ISTD revised technique, the IDTA (Walter Laird) and the UKA (Ken Akrill) all use the same description.
IMHO the change was an attempt to correct the musical interpretation which had developed as a result of bad teaching habits at beginner level.
In an attempt to get pupils to 'break on 2' many teachers repeatedly call the step timing 2,3,4&1 putting a very heavy emphasis on the 2 to the extent of shouting. This has the effect of making the 1 count seem trivial with the result that the step also loses impact.
The last step of the chasse is on beat 1 of the bar which deserves more emphasis. Keeping the first two steps of the chasse small allows this musical emphasis to be given to the final step.
Given a small step on 2, the difference between closing and half closing on the & is less significant so either can be acceptable, though I personally prefer a partial closure.
I also prefer to give my beginners a different emphasis when counting to accent the 1. I usually count a bar as 1,2,3, cha-cha instead of 2,3,cha-cha-cha, still 'breaking on 2' of course.
Change in musical styles may also have had an influence. One of the earliest chachas poular in the UK was Joe Loss's Wheels which used 1,2,3&4. Later South American influences brought us 2,3,4&1 and the modern heavy disco sound is open to almost any interpretation.