Being a non-engineer here at Cake (designer) and from what I understand, this happens in a certain combination of browsers and platforms. On Safari and Chrome on MacOS both title and post have spellcheck working fine for me. However, in other combinations the post may not have spellcheck working, while title does.
The reason is that we use two different text editors to power up each fields. The one powering the title doesn't support rich formatting (bold, italic etc.) and is much simpler. This makes it more compatible with a wider browser support. On the other hand, the post editor is much more complex (rich formatting, link expansions, mentions etc). This makes it less supported by some browsers and platforms.