I did a little playing:
it seems this is due to original OoB designer trying to cover a large period of time with 1 formation. The platoon formation is effectivley:
HQ section
Pre 1974 sec
Pre 1974 sec
Pre 1974 sec
post '74 sec
post '74 sec
post '74 sec
sniper
AT
ATGM
so the "gap" of 3 missing (out of timescale) units seems to create a further platoon.
I was able to prevent the problem by creating seperate pre-74 and post 74 platoon formations (plenty of room in french OoB
)