Uneven heating: bug or feature?

The sum of the heat in components is different than the heat in the hull.
Pretty sure the hull has only one temperature.
Can you give a specific example?

If I understand what you're saying right, that's intended behavior.

Though the reactor updates all its components once per second, in one batch, it does *not* update all its components 'simultaneously'. Each time the reactor updates, it goes through the reactor components in sequence, starting from the upper-left, and processing the whole row before proceeding to the next row. Thus, components that are processed before heating components will see a different situation than components afterward.

For example,
(Reactor Heat Exchanger) - (open space) - (Quad Uranium Cell) - (open space) - (Reactor Heat Exchanger)

When that reactor is first started, the heat exchanger on the left will see the hull at 0 heat, whereas the one on the right will see a 96-heat hull. As time goes on, the distinction matters less and less - as the overall heat grows, the small delta between the two exchangers becomes relatively small.

Overall, this behavior does drive a lot of reactor design, since being unaware of it can lead to some components 'mysteriously' melting while others have ample heat capacity left, etc. It's also why most (though not all) reactors put their uranium cells in the top-left corner, and any components that are supposed to retain heat in the bottom row - that setup leads to behavior that many find to be more intuitive. It's certainly not required to do things that way - and there can often be savings from organizing things differently - but it's a useful bit to remember.

Working as intended, but yeah, would be good to have all the facts written down about how the reactor and components react. Like the Uranium Cells distributes evenly between all surrounding components that can store heat, and such. Most people would learn this through experiments though, which is one of the fun parts.