That’s normal.
When they say, “increases by 25% per level,” what actually always happens is that it increases by 25% of the initial amount every time you fight that opponent. It’s crazy that they can’t say this more clearly.
It’s worth noting that it’s sometimes worth accumulating many copies of the same battle on the board at the same time, because they will always be at the same level, yet give increasing rewards (not this week, obviously). This only works for World Events where the whole board doesn’t get reset – or rather, only in between the resets.