The problem is that using the "Change Parameters" event command isn't working in the way you intended.
Stats are calculated as such:
Base Stat + "Change Parameters" additions
...where
Base Stat refers to the actor's stat at the level you specified it to be in the Database PLUS equipment bonuses.
So at the start of the game, the number in
"Change Parameters" additions is clearly 0. For your test, you set the base Max HP to 9999 and used "Change Parameters" to increase it by 9999, returning 19998 as expected. But there's a problem with
"Change Parameters" additions: it has a limit.
If you look in Game_Battler 1, you can find this method:
#--------------------------------------------------------------------------
# * Set Maximum HP
# maxhp : new maximum HP
#--------------------------------------------------------------------------
def maxhp=(maxhp)
@maxhp_plus += maxhp - self.maxhp
@maxhp_plus = [[@maxhp_plus, -9999].max, 9999].min #<============= This line in particular
@hp = [@hp, self.maxhp].min
end
The variable @maxhp_plus represents
"Change Parameters" additions. As pointed in the code, the maximum you can increase Max HP is by 9999 and the minimum by -9999. Any subsequent calls to increase the actor's Max HP will fail as @maxhp_plus cannot go beyond 9999.
The most band-aid fix for this is to replace the 9999 with
Config::Max_HP, like so
@maxhp_plus = [[@maxhp_plus, -Config::Max_HP].max, Config::Max_HP].min
However, be very careful with this. Say you have @maxhp_plus capped to 100000 (which is your limit you configured). Say you also have a status condition that increases the actor's Max HP by 5000 temporarily. @maxhp_plus cannot go beyond 100000, so it will ignore the additional 5000 being added to it. When the condition is removed, @maxhp_plus will decrease by 5000, meaning @maxhp_plus has now permanently lost 5000 points (and have your players in rage discovering the actor suddenly lost Max HP).
So to be on the safe side of things, it might be best to replace 9999 with something higher than your limit. And just be REALLY GOOD at keeping track of how much you are increasing/decreasing your stats with event calls.
(And I hope you are smart enough to realize I was only talking about one stat parameter and that you can easily fix the others yourself)