Friend: Normally it is done in a way that the candidate should be out of their comfort zone. However, if a backend developer would receive purely frontend work, then I would say that something went wrong.

That sounds like it’s just an excuse-generator so they can pluck out their desired hire without respect for any other factors.

The CTO’s answer was that they understood, they wanted to check if I was “a good fit for their culture” (of hiring backend engineers and assigning them unrelated frontend work)

This fish sucks at climbing our trees! (And of course, the “culture fit”.)

Until the workers are in control of hiring and firing, at least for their own teams, expect these shenanigans to continue.