Apparently since updating my SMBX version to the latest one (though I don't know if this is the causal factor for sure), I've run into more issues with this NPC – specifically, some body segments appear as head segments without apparent reason, even if they're at the top of the Pokey and the Pokeys otherwise work normally. Some of the Pokeys spawn normally, with a head at the top and the rest as body segments, and this is always consistent between repeated plays of the level: the one in the screenshot below always spawns with a head, a body, and then three heads for example. Here's an example of what I'm talking about:
Edit: After taking the screenshot I realized I should clarify, this is definitely not caused by the sizable behind the pokey, as it spawns to the right of the sizable and exhibits this issue right away. It still behaves as if it were completely intact, just some of the body segments look like/are heads.
Edit 2: Upon further investigation it seems that this is caused by not placing the pokey's segments in order from bottom to top in the editor. If they are placed from bottom to top, then the Pokey works normally. So there is a workaround, but it was still very unexpected behavior to me.