The alternative seatback model used for floor chairs is mismatched in size with their default setting, and the texture becomes flipped on both axis. There's also a visible missing texture line on the kelp version.

Default model (no issues):

Alternative model (line is hard to see, but certainly there):

Looking at the rear, you can see the size discrepancy in relation to the seat body of the chair. This discrepancy is not visible on the default model:

Thanks for the issue!
That size discrepancy is there intentionally to prevent z fighting. I don't know that it's identical between the two models, but it's present in both. It's a bit larger than it would otherwise be for compatibility with embeddium, which reduces the resolution of models.
Probably not going to make any changes considering how minor of a problem this is tbh