I am not sure.
they only said it differ for each unit.
for example, IS use eir as an example, a neutral eir get a +1 to HP,atk,spd.
other unit might get different result.
95% confident that the two stats that normally increase on the first merge get +2 instead of +1 and the third highest stat on the unit at level 1 gets +1. If there is a tie for third highest, atk is prioritized over spd is prioritized over def is prioritized over res.