Handshake deal between boss and employee for a bonus in exchange for doing the boss's work goes sideways when the boss suddenly leaves and the new boss refuses to uphold it: 'I was successfully fulfilling my role and extra work'

This employee discovered the hard way that vague promises of long-term reward in exchange for immediate contribution don't always eventuate when their boss left before seeing the deal through.

It's always said that a bird in the hand is better than two in the bush. If you haven't got something yet, it, simply put, isn't yours and isn't guaranteed to be yours.

In any situation its best to ensure that you have something concrete, or at least some written obligation that holds someone to a promise. Otherwise, all it is is some theoretical idea up there somewhere floating around in ethereal space, no more real than the memories of the two parties who agreed upon it. And especially in the workplace, regardless of what your boss's memory might be or what their intentions were, as soon as some resistance is met from their own superiors, they will likely go back on their word. Since going back on their word to you, their subordinate, is easier than changing the mind of their superior, as almost always will be the case.

Beyond that, a promise made by one person is very unlikely to be upheld by another, and a handshake agreement made between you and a supervisor is just that, it's reliant entirely on the working relationship between the pair of you to be constant and unchanged until it is seen through. And, let's be honest, that is very risky because nothing in this life ever remains unchanged or constant for very long.

Speaking of resistance, organizational management will always follow the path of least resistance; this means the things that are easiest to measure and track will be measured and tracked. And that these things, along with other initiatives and KPIs that are being prioritized by the organization as a whole, will be the things that result in "performance." Everything else will be pushed to the side in favor of concise, clean, and quantifiable things that constitute a job well done.

As a result, performance reviews can be narrow-minded. Covering gaps, preventing failures, and possibly contributing to a myriad of areas and an abundance of situations goes almost entirely overlooked.
While you've been busy patching holes and keeping the ship floating, you've actually been losing "points" in the things that matter and are tracked when it comes to performance.

And there is the clear recency bias to be considered, too. Anything that has happened in close proximity to the performance evaluation, good or bad, is going to carry much more weight, regardless of whether it is deserved.

This employee works at an international corporation where performance evaluations directly affect the bonuses they receive. With a good working relationship with their current boss, they were able to take on some of their boss's responsibilities, and in exchange, they received solid growth in their role and excellent performance reviews that offered them additional compensation. But when a new manager took over, he wasn't ready to buy the agreement and gave the employee only a satisfactory review, which meant that the employee didn't receive a bonus. Ironically, the employee had still been doing tasks that were the boss's own since the boss had joined the company.