Hello Doran,
we had a nice chat on Discord some time ago.
Eylau was a battle JMM was working on as a stand-alone title like Austerlitz.
The one you are playing is the community constructed battle.
Histwar N was and still is a BETA for the community, with major improvements
coming out in the near future
https://www.histwar.net/news.
HWN and its earlier Les Grognards both have issues regarding reinforcement arrival
(represented by 2 flags usually at each side rear center)
one flag can be placed on the flank of your deployment area but only up-to the border
with the middle ground.
There was no fix for this, and I'm not sure if JMM has fixed it to be included in the patch
to come.
The stand-alone battle of Eylau (mini-campaign) was a title he had announced
was coming with the new improved HWN engine and new graphics.