Interesting point.
I expect Monzo will continue using this model of collecting some of their partner’s referral incentives, which may sometimes mean that the user receives a smaller incentive, in the future - assuming that this test & others work. We also know that they’ll be transparent about the incentive (the £25 in this instance), that they’re receiving for promoting their partner.
So lets assume that Monzo’s testing whether or not losing £25 of the incentive puts you off signing up through them. If they’d told you that they’re collecting £75 (the £25 you’re giving up + the £50 that Bulb gives referrers) & you’re collecting £25, how would you have felt about that?