Elaborating on an issue that Livia noticed on the discord. Looking at the ambition code myself, I can easily see the problem, too:

https://github.com/ck2plus/CK2Plus/blob/main/CK2Plus/common/objectives/000_CK2Plus_ambitions.txt#L3354C3-L3363C4

if = {
	limit = { has_character_flag = obj_win_a_war }

	prestige = 25
	set_character_flag = obj_win_a_war
}
else = {
	change_martial = 1
	prestige = 50
}

That's the two if blocks starting from line 3354 of that file. The flag that it's looking for to give the lesser reward for finishing the ambition is only set if the character already has it?