It's not a bug, its a property of the colour space. Which is partially tied to how the colour is represented (RGB). When doing linear interpolation through the RGB cube (for eg a gradient), you normally pick the shortest path. It just so happens that sometimes that path passes thorough some shade of gray as different colour components are scaled.

Usually you fix it by moving your point through a different colour space. Choice depends on your requirements and mediums you're working with (normally different types of light sources or screens).

I had to write a low level colour interpolation librar for a few interactive art projects, so I dipped a bit into this, but I'm no colour expert

No, sRGB refers to both a colour space and an encoding of that colour space. The encoding is non-linear to make best use of the 256 levels available per channel, but you were never supposed to interpolate sRGB by linearly interpolating the encoded components: you're supposed to apply the transfer function, perform the linear interpolation at higher precision, and then convert back down into the non-linear encoding.

Failure to do this conversion is what leads to the bad results when interpolating: going from red to green will still go through grey but it should go through a much lighter grey compared to what happens if you get the interpolation wrong.

I think GP is referring to the difference between "normal" (gamma-encoded) sRGB and linear sRGB. Though it's not logarithmic but a power law. In any case linear interpolation done in non-linear sRGB gives you intermediate colors that are darker than they should (though historically it's been so common in computer graphics that people are accustomed to it).