I asked a candidate to implement an algorithm that reverses a string. I don't see that as a complicated problem. It's literally a for loop, and based on how they choose to implement it you can ask about implications of append vs. prepend, etc.

I didn't ask them to implement dijkstra's algorithm, or even something as "hard" as breadth-first or depth-first search (which I feel any senior that deals with trees should be able to do).

It's a marker of this... Can the candidate solve a basic problem? Once they solve it, can they explain their solution? Do they understand the memory/time tradeoffs they chose? Do they know whether the code they wrote is thread-safe? If you don't feel a senior engineer should be able to answer those questions, to a very basic problem, how do you expect them to tackle something more complex?