Yes that was it! There was indeed one keyframe in an unrelated smartbone. Thanks Victor.

So if I want to use target bones, should I avoid adding keyframes to smartbones in general? Is it a bug? Or is there a workaround for this?

Hey all, long time Moho user here but new to the forum. I have a question about target bones: they seem to be working fine for me on frame 0 (Design Mode), but on any other frame the rig doesn't respond to them at all. Any ideas on why this is?