You are correct. I've heard it both ways and ascribed slightly different meanings, but deep-seated was what I meant.

It's an eggcorn!

Oxford, Cambridge and Merriam-Webster say no.

Why do you say this? And why phrase it so tersely?

Merriam-Webster at least (the only one I've checked) says that "deep-seated" is the only correct version:

  Definition of deep-seated 
  1 : situated far below the surface
  2 : firmly established
  https://www.merriam-webster.com/dictionary/deep-seated

  “deep-seeded”
  The word you've entered isn't in the dictionary.
  https://www.merriam-webster.com/dictionary/deep-seeded

I was trying to make it clear that whiddershins was wrong to say that ‘deep seated’ is the correct usage in contrast to ‘deep-seated’ when Oxford, Cambridge and Merriam-Webster say that ‘deep-seated’ is indeed a correct usage.

P.S. Inconsistency: I assumed that whiddershins was complaining about the hyphen, rather than correcting ‘seeded’, which had been changed to ‘seated’ by the time I first saw it.

I see. You were pointing out the hyphenation or lack thereof? I think you might have missed that the earlier discussion was regarding "seated" versus "seeded". Thus your brevity made it looked like you were claiming that the "seeded" form was correct. I don't think anyone else was concerned about the hyphenation.

Now we have three variants in the wild. The correctly folded "deep-seated" and its harmless but slightly anomalous "deep seated". Worryingly, there's this new configuration "deep seeded" that a significant but unknown number of HN readers may have been exposed to. Although the original infection source has been neutralised, we must remain vigilant against sporadic recurrence in future.

Yes, I was indeed concerned by my wrongly perceived concern of whiddershins over the parent’s usage of the hyphen.